If you're looking to ski just the afternoon, you can get a half-day ticket at almost all the resorts. Mt Sunapee is less than 1 1/2 hours from Boston and they charge $36 for a half day lift ticket. Their terrain is pretty good, but the slopes are definately crowded.
With the lack of snow we have had though, I don't recommend skiing in the afternoon unless you are a beginner. The afternoon slopes (blues & blacks) are going to be very icy because they will be tracked and the snow will be pushed.
You can get a half day (8 - 1:30)morning ticket at Okemo for $53. But it's a 2 1/2 drive from Boston - but definately worth it. I think I heard a radio commercial that said Wachusett does a half-day morning pass too. They're less than an hour from Boston, but I've never been so I don't have any details.
Cannon is 2 hrs from Boston and has afternoon half-day passes for $32 or full day for $45. Cannon is a great mountain for steep terrain. The only problem is that it's a very cold mountain, so check the weather first if you decide to go there.
Have fun [img]graemlins/thumbsup.gif[/img]