THAT GREAT BIG building beside the Aquarium in Baltimore's Inner Harbor used to be a power plant. Now it's the Power Plant, a sort of giant funhouse created by the Six Flags theme park people.

The building itself is almost worth the price of admission, because it was a handsome thing when it was built at the turn of the century and the remodelers have handled it with care. The boilers and dynamos are gone, of course, but the mighty smokestacks remain, and the intricate iron skeleton of the structure delights the eye.

It's a cool and inviting oasis of shows, sideshows, food concessions and shops, all run by unfailingly cheerful young people. Six Flags describes the Power Plant as "an urban entertainment complex," rather than as an indoor amusement park, because there are no rides.

No expense was spared during the $25 million project, the company says, and the place certainly sparkles. Yet there's an almost perfunctory quality to the four shows that are the core of the attractions. All are very brief and some use special effects that are nothing special, such as laser holography that's not as good as they were doing at Disney World a decade ago.

The shows, which patrons are entitled to see only once per Power Plant visit, include the Magic Lantern Theater, an "Animatronic" show made up of mechanical dolls that perform everything from circus acts to Gilbert & Sullivan songs. Unfortunately the conception is as mechanical as the execution. And there's an egregious lapse of taste at the entrance, where there's a framed letter, supposedly from John Wilkes Boothe, that says if actors are to be replaced by mechanical performers, he's "likely to do something I'm ashamed of." Ho, ho.

The unifying conceit of the remaining three shows is that the Power Plant is a monument to the genius of Phineas Templeton Flagg, a mythical Baltimorean supposed to be the contemporary and peer of Edison, Bell and our other great inventors.

* The Circus of the Mysterious is a raree show of wonders Flagg gathered on his explorations, including a pretty good sinking ship in a bottle, a fountain of youth made to appear to flow backward by varying stroboscopic flash rates, and Pandora's box. Best of all is a dancing leprechaun, who shows what can be done with holograms.

* Laboratory of Scientific Wonders, which features an "automated home of the future" that you've probably seen and not enjoyed before; a "Power Core" chamber, which they warn you may frighten small children but may not even wake the baby, and a brief look at a neat model of what a Victorian seer might have expected Baltimore to look like a century hence.

* The Sensorium, best of the four shows. It's a 3-D plus smell-o-vision plus vibrating- seats movie that's fun but ftful. It flits from scene to scene with little continuity, and doesn't end so much as just stop.

Some of the problems with the shows will no doubt abate with fine-tuning (the Power Plant's only been open a couple of weeks), but even so they seem unlikely to draw the repeat business a year-round amusement center would seem to need.

The sentiment of our party was that we'd like to go back if you could get in for a couple of bucks and skip the shows. In that case we could repeatedly enjoy:

* Good food (including salads and fruits and other light fare) at decent prices in the cafeteria and great diet-destroying snacks and desserts such as ice-cream waffle cakes and candy apple slices at stands throughout the building.

* The Skeeball, video games and other skill amusements.

* Just cruising the building itself, as do strolling performers who contribute much to the festive atmosphere.

POWER PLANT -- Open 10 to 10 daily. Admission: $7.95 adults, $5.95 children four to 11, under four free. At Pratt and Light streets (next to the Aquarium) in Baltimore's Inner Harbor. 301/244-7377.