IT'S TUESDAY MORNING but it feels like Saturday. For me, Tuesday's the beginning of my "weekend." I work Thursday through Monday and am always looking for something new and different to do in the middle of the week.

Weekdays might not be the conventional time for the normal nine-to-fiver to play, but that's what makes them so great. You'll find smaller crowds, more parking and shorter lines.

There are places in Washington and the suburbs that will feel like a ghost town on some of the most beautiful summer and fall days.

Find a friend to play hooky or go solo.

If you want to play tourist or happen to be one, head for those places that only offer weekday tours. But these places are usually crowded; if you're looking for some solitude you might head for the suburbs or at least avoid the Mall area.

Those who cross the 14th Street Bridge to work are well aware of the popularity of the Bureau of Engraving. Lines form early for the tours, offered from 9 to 2 Monday through Friday. Here you'll see how easy a buck is made. The line starts at 14th and C NW. Call 566-2000 for more information.

The FBI gives tours from 8:45 to 4:15 Monday through Friday on E between Ninth and 10th NW. This tour includes exhibits, history, a forensic laboratory and a live firearms demonstration. Call 324-3447.

The Library of Congress offers tours from 9 to 4 Monday through Friday on Independence at First SE. Call 287-5000.

If you're interested in the workaday workings of a newspaper, tour The Washington Post Mondays and Thursdays, by appointment. You can work your way up from pressroom to newsroom in a matter of minutes. Call 334-7969. The Journal Newspapers offer weekday tours, by appointment, of their state-of-the-art plant at 5883 Commercial Drive, Springfield; call 750-8190.

If you don't want to go far but also don't want to get caught up with the tourists, the George Washington Parkway (Virginia version) is a good place to start. If you are starting out from Washington you can pick up the Parkway from all of the Potomac River bridges (in the case of Chain Bridge, you'll have to turn right and go north on Virginia Route 193 for a mile or two).

From downtown, follow the parkway north for the more spectacular view of the city. As it ascends along the river as you pass Georgetown you can catch a glimpse of the university and some beautiful homes. The drive is stunning in fall when the leaves are turning.

Just as you exit from or pass under Roosevelt Bridge is the entrance to Roosevelt Island, 88 wooded acres opposite the Kennedy Center. It's a great place for a picnic, birdwatching or some light fishing. Head for the south end of the island just under the Roosevelt Bridge and catch some small perch, catfish and bass. All birds common to the area can be seen on the island, including some water fowl.

Continuing north, exit the parkway on Route 123 towards McLean then pick up Route 193 west for seven or eight miles until you come to Great Falls Park.

Great Falls has the most famous of all views of the Potomac, and the water-carved granite walls of Mather Gorge draw hordes of rock climbers. The Patowmack Canal, built along the gorge by George Washington, opened in 1802 to bypass Great Falls. The remains of the canal can be seen downstream from the visitor's center near the holding basin. It's a great place to take out-of-towners and an even better place to take a new romantic interest.

Also the vague remnants of Matildaville -- the company town built along the canal where many of those who worked on the project lived -- can be seen near the holding basin.

River Bend Park is the first right past Great Falls off Rte. 193 and offers fishing, boat rentals and picnicking.

Instead of going north on the parkway from the bridges, you can go south to Gravelly Point, just north of National Airport (from the southbound parkway lanes you'll have to take the airport exit and loop back north). If you get a thrill from the sensation of huge, incredibly noisy airplanes landing almost on your head, this is the place to get that thrill. Nearly a million people a year do.

Continue south on the parkway five miles past Old Town Alexandria to Fort Hunt, which was originally established during the Spanish-American War and was in operation until the late '40s. Now it's Fort Hunt Park, and all that remains to remind visitors of matters martial are four gun batteries that make for great entertainment for small children. It's open during daylight hours, 285-2598.

Overlooking the Potomac near Woodrow Wilson Bridge you can get back to nature at Oxon Hill Farm. There you may milk cows, collect eggs, pick apples and shear sheep in season. This 485-acre, 1900s working farm is on Oxon Hill Road. Visit 8:30 to 5 or call 839-1176.

Follow the parkway to the end and visit Mount Vernon, home of George Washington.

The crowds are pretty constant at Mount Vernon, but if historical homes are your cup of tea, there are plenty to see in the metropolitan area.

Visit Cedar Hill, home of Frederick Douglass, the runaway slave who became one of the most eloquent and effective of abolitionists and the first black U.S. Marshal (of Washington). He bought the house at 1411 W St. SE in Anacostia in 1877 and lived there until his death in 1895. Tours are 9 to 5. Call 426-5960.

The Clara Barton House -- its owner was the founder of the American Red Cross -- offers 38 Victorian rooms with tours given by guides in turn-of-the-century dress. The house became the first permanent headquarters of the American Red Cross. Tours are 10 to 5 daily. Call 492-6245.

The house on the hill overlooking Arlington Cemetery is the "Lee Mansion," or "Arlington House," or the "Custis-Lee House," or whatever. Robert E. Lee and Mary Custis married there in 1831 and called it home for 30 years, having six of their seven children there.

The Lees left for good when he headed for Richmond to become military adviser to Confederate President Jefferson Davis.

The easiest way to reach Arlington House is by subway. Take the Blue Line to Arlington Cemetery, then walk between the crosses row on row or take a tourmobile to the house. Tours are given 9:30 to 6. Call 557-0613.

If you enjoy the turn of the century, you might enjoy a mule-drawn barge ride on the C&O Canal. This hour-long trip on the 90-foot canal boats is offered at two places on the Canal.

In Georgetown board the barge at Foundry Mall, 1055 Thomas Jefferson St.; in Great Falls at the Great Falls Tavern in Maryland at 11710 MacArthur Blvd. In Georgetown call 472-4376. In Great Falls call 299-2026.

Back in the city, Rock Creek Park offers 1,700 acres of bicycle paths, horse and hiking trails, a nature center, tennis courts and a golf course. And every now and then, if you're lucky, you'll see one of the deer that wander in from Montgomery County from time to time.

During midweek the bike path is relatively empty at midday along the 7 1/2-mile stretch from the Maryland line to Memorial Bridge, which connects the bike path on the George Washington Parkway.

Rent a horse at the Rock Creek Horse Center. Call 362-0117. The golf course is at 16th and Rittenhouse NW. The nature center offers tours and a planetarium at Military and Glover NW. The center is open 9 to 5 Tuesday through Sunday, (but will be closed two months for repairs, beginning this Friday) 426-6829.

There's nothing like an empty mall or movie theater if you're looking for a place to hide from the dog days of August.

The Circle Theater chain offers half-price admission Monday through Friday to the first show at all theaters (except the Uptown). Most of the chains offer reduced rates for either the first show or all daytime shows on weekdays.

Drive out I-270 to Lakeforest Mall in Gaithersburg. You can get much more personalized attention during the week from the salespeople and you'll have your pick of parking. The mall, which is open and airy, has 150 stores -- including Hecht's, Woodies, JCPenney's and Sears -- restaurants and services. Most of the suburban malls are relatively empty during the week, especially in warm weather.