The problem with Zipcar for this kind of expedition is that you're still paying for the car the entire time it's sitting in the parking lot, so that's going to be what looks like a $68 daily rate just for transport on top of the lift ticket and ski rental. Wachusett's big draw in general is being imminently driveable from Boston, though, it's not a bad drive. I suspect that for a 2+-hour time commitment you could actually make it up to something more plausible in New Hampshire.

The Wachusett-oriented summer activity is the Charles River Wheelmen's Climb to the Clouds century, modulo ukelele's terrible experience with losing a spoke at the top of the mountain and somehow having lost the ride trailers.