Mountain biking at ski resorts is a cool option, particularly when you can ride the chair lift up with your bike and enjoy an adrenaline rush ride down. Just be warned that green circle ski trails are more like black diamond descents on a bike - the surface is not soft carve-able snow - its dirt and rocks, grass and gravel. A full suspension bike is recommended and riding experience is essential for big mountain biking. See Heather's article on Sunday River's mountain bike park that offers 30 trails, lift accessed.
Hiking is another option, and ideal with a chairlift ride, up or down - your choice. Again, hiking bots are better than sneakers. You will be surprised how much steeper and uneven the trails are without snow cover. The reward is seeing how long, scenic and sprawling the mountains are in summer.
Ziplining is one of the coolest 21st century add-ons at ski resorts. The chairlift brings you to the summit, or at least up the mountain, where you harness in and zip down a cable from platform to platform with amazing views and a rush of a ride. I love Gunstock's zipline in New Hampshire - you control your own speed, and the scenery of Lake Winnipesaukee is amazing!
Okemo in Vermont has a zipline tour and a Timber Ripper mountain coaster - which is like a mini-railroad cart that swerves down the mountain on a track... very fun.
So don't dismiss the ski mountains in summer - there's plenty of fun pastimes and pretty spectacular scenery too! Ski season will be here soon enough!