most places will do a vegetarian dish upon a request (if you don't like what they already offer -- i cannot recall the last time i was someplace that didn't offer at least one) and there are a gajillion middle eastern and indian places whose menus are 50% if not more vegetarian. french and Italian places also do well with veg. i was a vegetarian for years and never had a problem dining out.
there is also grezzo, which is 100% raw, on prince st., but that is pretty spendy.
are you hankering for something special cuz your op is pretty vague.
Cafe Barada, just outside of Porter Square, (77 bus, maybe?) always has a veggie special from their rotating menu, as well as falafel, yummy mujeddara (seasoned lentils and rice), and delicious ful mudammus (brown beans with lemon, garlic and olive oil). Really nice people, too.
Not vegetarian or marketing itself as "healthy", but Henrietta's Table uses tons of fresh, varied (and local) produce, especially at lunch time. I had lunch there last week and was pleasantly surprised to find 4 or 5 interesting salads on the menu. (I had the arugula, nectarine, goat's cheese, spiced pecan salad special.) There are always a couple of vegetarian cooked entrees too. That's where I go when I want a broad, tempting selection of vegetables, without any mystery ingredients. It's nice to find that minus an atmosphere of deprivation.
Davis Sq: Martsa on Elm (Tibetan) and Chipotle have vegan dishes.
Harvard Sq: Veggie Planet and Asmara (Ethiopian/Eritrean) also have vegan dishes
Porter Sq: (or 77 bus) Elephant Walk (French & Cambodian, which also has locations in Boston and Waltham) and Boca Grande have vegan dishes.
Haven't been yet, but on 77 bus line in East Arlington, Kathmandu Spice appears to have vegan dishes and Flora has a new veg tasting menu (but it's on hold during restaurant "week" which apparently lasts all of Aug).
Not too far from the commuter rail line in Waltham is Karibu (Ugandan) has vegan dishes. Also in Waltham but farther from the station is Masao's Kitchen.
Do you know about happycow.net?
Depending on how much walking you want to do...there's Life Alive in Lowell's...we walk there from the commuter rail station.