double tree is on the same side of hbs, 2 miutes walking. the days inn is about 20 minutes walking to hbs. but you can take bus 70 (there are a lot during the day) and it will arrive the entrance of hbs (western ave side). as i know there are now better choice nearby. Good luck
save money? bid on priceline to get a hotel (hampton inn/holiday inn) near the airport, i paid 50/night in 2005. both have free shuttle service to/from airport and t station (blue line). take t to harvard square, walk down to the j.f. kennedy st. and cross over the charles river to get to the hbs.
don't rent a car, driving in boston could be a nightmare to first time visitors.