Veggie Galaxy is unique, with really good food and a fun atmosphere. It's across the river in Cambridge....about a ten minute drive unless the traffic is bad. But you could also leave your car at Mass Eye and Ear and take the Red Line to Central Square, which is only two stops away. Then go back again. That's what I would recommend if you go to Veggie Galaxy. Their menu is on line. Check it out.
Can't help much for right in the hospital region, but here are two that are a short and moderate clip away (1 and 2 subway stops with the first one being walkable). Both are on the casual, kid-friendly side but certainly aren't foodie places.
For casual and fun, cross the bridge into Cambridge and go to the Friendly Toast. Lots of vegetarian and some vegan options:
Further into Cambridge in Central Square is one of my favorite veggie restaurants lately, Veggie Galaxy (a sister to Didi Emmon's Veggie Planet). It's on the more healthy side of diner food: