Harvard Square is nowhere near as cool or fun as it used to be before the developers raped it, but there are still a lot of cool stores and things to see there. Davis Square is always a lot of fun. The Museum of Science is a good option if you like that sort of thing, and their IMAX theater usually has good movies. There’s a neat little MIT museum that used to have a lot of holograms; I don’t know what they have now.
The Arnold Arboretum is beautiful, if you like to walk. There’s a secret place there that’s simply amazing; my son and I call it the “Magic Tree”. Here’s a link to a post on my blog with pictures and directions: http://bobquasit.livejournal.com/537933.html
Oh, and a walk down Newbury Street (from the Mass Ave end) to the Public Gardens is not to be missed in nice weather, particularly if you’ve ever read “Make Way For Ducklings”. The Maparium at the Christian Science cathedral is also very cool, and I believe it’s still free.
Basically there are a million neat things to do in the Boston area. Different things are happening all the time. You might want to pick up a copy of the Metro (a daily free paper); they always have a daily listing of fun stuff in the area. The Boston Phoenix (also free, but a weekly) also lists lots of stuff.
I’m not a vegetarian myself, but I’ve been reading rave reviews of Buddha’s Delight for at least ten years now. http://www.google.com/search?hl=en&q=Boston+Buddha%27s+Delight
Hey, that reminds me: Chinatown! That’s a pretty cool place to walk around. So’s the North End, if you like Italian food and ambiance.