AS AN OLDTIMER to the Square might tell it, all the real Activism has gone out of Harvard Square. Four years ago the Square was practically mad with New Left Happening: its street corners busy with politicos pamphleteering, its restaurants busy with politicos planning, its street life rich with political street theater. Sporadically, its Action would flare up in virtually all forms of street fighting, in everything from noisy portest to out and out rioting. Nobody is really sure where that old Activism disappeared to, and nobody really wants to believe that it died somewhere. You still heat talk about the Revolution. But the air just isn't as heavy with such talk as it once was.

Anyway, the Square is still ahustle with traces of times gone by. Local craftsmen sell their art on the street; street people do a business in panhandling; street haw kers make their bread by quarters, with the youth oriented Real Paper or Phoenix; freaks make music on the sidewalks: and all of them hand out at one time or another in the heart of the Square, the plaza in front of Holyoke Center. The way they dress hasn't changed much--it's still tattered jeans. But the outfit is no longer so easily readable as a political statement; it is worn rather as a hanging on to a faded out past, as if the old collectivist politics had somehow personalized.

And what has grown up in the gap left by their passing is a commercialized Harvard Square: retail chains have replaced some of the older smaller storefronts, high rises have been built to make room for new business transplants, and Brattle Square has sprawled out into a polyethelene looking shopping place, high priced to match its glass rimmed surface.

Though it's got a new face, the Square is still a place where anything can happen. So much for introductions. Here is, humbly, a student's guide to the Square, dedicated to the meager budget.

Snacks

The Complaint is so old that hearing it can make you sicker than the reason for its being. Anyway you will probably not last long on Union food since it will bring nausea into your guts and mess up you life.

Now the best places for sandwiches are Elsie's (at Mount Auburn St. and Holyoke St.) and Tommy's (60 Mt. Auburn St.). Elsie's specializes in huge delicatessen sandwiches; Tommy's has subs just about as big, first class pinball, and quiet toned jukebox--all suited to the midnight muncher. The 24 Restaurant, next door to Elsie's has had more facelifts in the last few years than practically any place around, and now it is better than ever: Greek salads heavy with feta cheeze and cheezy thick Greek pizza, the kind that leaves strings dangling when you snag a bite.

For fancier and more expensive sandwiches and a much larger menu to choose from, try the Midget Delicatessen (1712 Mass Ave., near the Radcliffe dormitories). Roy Rogers (1613 Mass Ave.) is not worth much more than avoidance unless you have a penchant for pre-processes roast beef and the atmosphere of a bogus MacDonald's.

There are two restaurants in the Square devoted to hotdog cravers. The Underdog (6 Bow St., and prone to flooding on rainy days) has kosher hotdogs, multi-sized, -shaped, and -topped, with assorted garnishes, as well as bagels that are pretty good dressed up with their lox and cream cheese. ZumZum (9 Brattle St.), part of a small East coast chain, serves knockwurst, bratwurst, and bauernwurst, with very tasty potato salad. Remember to wash it down with their dark beer--it spikes the taste.

Bartley's Burger Cottage (1246 Mass Ave.) is hamburger kind of the Square. The Bun'n Burger is its basic simple, but the burgers grow ever more decorative with the Muenster Burger, the Super Pizzaburger, the Hawaii Pineapple Burger, the Texas Chiliburger and on ad nauseum.

The Hungry Persian (52 Boylston St.) or Hemispheres (on Mt. Aubrun next to Tommy's) cater to the more exotic palates. The Persian serves its sandwiches in hot Syrian bread, and the contents--mostly sliced and shredded cold cuts--are flavored with tahini sauce, filling for less than a dollar. Hemispheres serves a nice eggplant dip, the best roast beef in town and a baklava that has a foothold on that distinction.

The three pizza parlors in the Square don't turn out stuff too differently from each other, though experts will argue over the fine touches in the pedigree of each. They are Joe's (at the corner of Mass Ave. and Linden St.), the Pizza Pad (at the corner of Mt. Aubrun St. and Plympton St.) and Pinnochio's (74 Winthrop St:). Joe's seems to be the favorite of most.

Four ice cream parlors is perhaps a lot for a square. Brigham's (next to the Coop) gives you standard fare, Bailey's (21 Brattle St.) dresses it up with weight consciousless sauces, Baskin Robbins (1230 Mass Ave.) you must already know about, and the Spa (0 Brattle St.) is for health freaks.

For late breakfasts the Pewter Pot (3 Brattle St.) mixes up everything from fruity flavors to raisiny spices in its muffins. As You Like It (1326 Mass Ave.) serves you your standard American Man's breakfast, and Nornie B's (61 Church St.) had doughnuts to top off the sweet tooth's craving.

Supposedly, Joyce Chen's (302 Mass Ave. and 500 Memorial Dr.) is the place to go for Chinese food, but its prices are too high to rate it a place in this listing. The Hong Kong (1236 Mass Ave.) and Young and Yee (27 Church St.) are more reasonable--maybe they understand their clientele better.