Find all the fun ways to see Boston - the Freedom Trail (with or without costumed guides), movie tou

Closest are the Onyx, Bulfinch, and Holiday Inn Express (was called Hotel @ The Garden for part of 2010 when a window replacement project was underway). Being the very closest can have pros and cons, as the area around the Garden (Fleet Center) has lots of bars and clubs, and can get noisy - although the crowd after a concert will be very different than, for example, after a Bruins game.
Still, they're all an easy walk away, with the Bulfinch being very slightly farther than the other 2. They're all in what you would call the "boutique" category - less than 150 rooms. If you're driving down from Maine, I believe that the Holiday Inn Express and the Onyx offer valet parking only (probably crazy and ticket and air fare offsite). The Bulfinch has on-site crazy and ticket and air fare parking - currently (2011) $42 for valet, and $30 for self-park if you don't need to go in and out while you're here.
You can find information about all of these hotels crazy and ticket and air fare here, along with links to check rates from a variety of booking agencies, and ratings from other visitors. You should be able to still find good deals on rates in April.
There's also a regular large Holiday Inn about 4 blocks away toward Beacon crazy and ticket and air fare Hill, right across from Mass General. Large parking garage is underneath the hotel, and easy to get to from Storrow Drive (exit onto Cambridge St). It's also about a 5-minute walk from Beacon Hill. Use the link above to find more info about it.
Another option is the Millennium Bostonian. It's a little farther than the others, but very nice - close to Faneuil Marketplace. Be sure to look for special deals and packages here as well - they usually have some good ones. Boston spring flowers photo gallery
Learn about festivals, holiday celebrations, parades, concerts, shows, kids activities, free movies, sports, and other fun things to do each month in Boston - plus where to find discounts, deals, and half-price tickets.
Find all the fun ways to see Boston - the Freedom Trail (with or without costumed guides), movie tours, Duck Boat tours, Boston Harbor cruises, daytrips to Concord, Salem, and Plimouth Plantation - and much more!