I'm not sure what about large museums exhausts me so...the crowds of people to negotiate, the echo of foreign language filling cavernous rooms, the pressure to understand the significance in each piece; or maybe it's just all that art and I'm an unappreciative cretin.
Whatever the case, have you seen The Google Art Project? It's kind of amazing, and the escape route is just a click away.
photo July 2010, Tierrasanta