summer set ups, while they can be sketchy if you dont set it up right, are a really good thing that are a lot of fun. trampolines are good, or in the summer go to a local pool/lake and use the diving boards. slacklining is a lot of fun to get balance down (set it up between two docks over water for more fun).
other than that, strength/endurance training and stretching if you are serious about it and want to avoid injuries. the above things are good for air awareness, balance, and core, but you still need strength so you dont get hurt. lots of biking (i've been going 20 miles a day), as well as squats (one legged on a balance ball works really well) and box jumps are my favorite. also do lots of core work, things with medicine balls seem to work really well. and dont forget about upper body. while it may not be as important to skiing as legs, it is still important to being well rounded and in over all good shape, and will help (ive popped my shoulder out 2 times this season, and probably could've avoided taht if i did more upper body last summer)
also stretch a lot. lots of injuries in ski season could be avoided if people stretched so their bodies could take more abuse. i normally just do stretches in the morning, after i work out, and at night (if i didnt just work out). some people like yoga. i dont doubt it helps a ton, and ive considered it, but i dont think its for me personally, although i might to get ready for next year.