While the rest of us are content to settle for sit-down dinners or the occasional catch-all cocktail party, a 30-year-old former securities broker has decided to put lilt into life by giving a series of parties that require a bit of imagination.

John Ritchie, founder and sole proprietor of an entertainment company called Completely Different, Ltd., thinks that most parties are too staid and that career professionals often are too busy to organize anything exciting.

"I decided to set things up for them. I'll give them something different. People have to participate in my parties. If they just sit back, they won't enjoy themselves. They have to be willing to be crazy and take on a role or it won't work."

Part of Ritchie's first Completely Different party can still be felt underfoot on the floor of his Capitol Hill town house--little gritty drifts of sand left over from one night last February when Ritchie gave a beach party. He covered the floors with plastic and covered the plastic with six inches of sand and covered the sand with 300 to 400 friends who showed up driving convertibles and wearing shorts and T-shirts to build sand castles, drink beer and dance barefoot on the instant beach.

A good time was had by all and soon a good time will also be had by the neighborhood children, whom Ritchie is inviting to bring their sandbuckets--for a kiddie clean-up--to the vacant lot where he's dumped the sand.

Now that the sand is out of his shoes, Ritchie is busy planning his next fantasy party, open to the public, date still undecided. For the "Einstein-Moet-Lorentz Relative Movement Space Time Continuum" he will take over a bar and lead guests into the future with computer games, displays, demonstrations of home computer equipment and the antics of a 2 1/2-foot-tall robot, which, Ritchie assures, may be "verbally abusive, but will present no physical danger to the clientele."

Guests will enter through a decontamination chamber and there will be music, costumes, a "Nutri-Matic Drinks' Synthesizer" and a "Pan Galactic Gargle Blaster" described by Ritchie as "the alcoholic equivalent of a mugging."

Putting together his Completely Different parties, translating an idea into the reality of finding a location and arranging for decorations, licensing, insurance, etc., has made Ritchie, a native of Philadelphia, understand why most people settle for standard-brand entertaining.

When he announced plans, for example, to cover walls of the bar with black cloth to set off the futuristic designs, his insurance company balked. And so the cloth will be made fire-resistant.

The event Ritchie is planning for fall will even demand a parade permit. The D.C. Area Chapter of the Great Cross Country Kinetic Sculpture Race is modeled on a 32-mile race first held in California 12 years ago and now an annual event worthy of that state's reputation for nuttiness.

"The object of the race is to complete it with the utmost silliness and maximum craziness while so doing," as Ritchie explains the local version in his brochure.

"A kinetic sculpture," he adds, "is any vehicular work of art that can be pushed, peddled, pulled, paddled or otherwise humanly moved . . . Who cares about being first--just get that wonderful heap of junk across the finish line in style . . .

"Spectators have a definite obligation to add to the silliness with horns, cameras, properly disrespectful hand signals and clothes . . . official race T-shirts, race caps and white gloves."

In the future are the "Vahalla of Venison and Wenches at the War Lodge of Thor," a Viking dinner with appropriate foods and a carefully choreographed sword fight, a "Pythonesque Memorial Silly Walk & Upper Class Twit Obstacle Marathon," which Ritchie is thinking of holding down at the shore this summer: "a chance to be really silly."

The "Callous & Crude Classic Comedy Compendium" will feature classic comedy film clips and at the "Dreaded Dragon and Damsel Design Debauche" guests will be asked to wear Renaissance costumes, do the old dances and perhaps slay a dragon or two.

Ritchie's ideas and the way he plans to carry them out require a lot of advance preparation and lots of participants. How could you possibly have a satisfying kinetic sculpture race with only six or eight people? Which is why, he points out, "People can come to these parties and do things they couldn't do on their own, things that are different and that I can organize for them.

"When people become physically involved they lose their self-consciousness. If they meet other people at these parties, great. But my purpose isn't to put single people together. It's to have parties for people who enjoy doing crazy things."

Completely Different, Ltd., parties for members vary in price from $10 to $15, including food and drink; $15 to $20 for non-members. Members also receive a monthly newsletter with information about upcoming events. A single membership is $25; couples, $35.

The next Completely Different party will be an outdoor barbecue preceded by a scavenger hunt April 24 at the Turkey Run Farm in Virginia. For members, $5; nonmembers, $10.

For further information, call John Ritchie, 543-8116, or write, 1229 Massachusetts Ave. SE, Washington, D.C. 20003.