Depends on where you are. From MA, Monadnock is probably the closest mountain with a great view, but it will be super busy there. If you're willing to drive further to Franconia, Mt Pemigewasset has a great view and is an easy hike. And another 20 minutes past that, you can go to Middle and North Sugarloaf. Again, easy hike to either peak. Middle has the best view, but North's is also fantastic.

If those are too far, the lakes region has a few less spectacular mountains, but still great views. Belknap has a firetower that let's you see to Boston on a good day. Gunstock has a gondola and a store with a viewing deck on its peak. Nearby that is also Mt Major, another popular one with a view over Winnipesaukee.