Another vote for the 15-22... look for the MOE version if you can find one at a good price. I have the MOE and it is a very fun training gun and accepts most AR15 accessories. The MOE gets you MagPul flip up front and rear sights, the MOE stock and pistol group, as well as a QD sling mount integral to the lower (the other MagPul gear can be added, but the integral QD cannot be retro'd onto a standard 15-22). Usually the price difference between the MOE and standard versions is not that significant compared to the amount of extra's you get.
I added a Primary Arms reddot on mine which increased the fun significantly as well.
I added a Primary Arms reddot on mine which increased the fun significantly as well.