I like doing a little liberty work with my yearling. She trots circles around me and we can do halterless showmanship and stuff. It's fun, but just something she picked up naturally along the way.
But I would caution against doing tricks with a yearling. I don't know much about what kind of tricks you are doing, but yearlings are annoying enough when they aren't trying to "smile" at you all the time for a treat.
I would also not teach them to rear, for obvious reasons.
But there's also no harm in turning them out to grow up if they know everything. My 13 month old is show broke and a great longeliner and showmanship horse. But after worlds when she is 14 months old I'll be turning her out until she is 2. If they've been trained solid they'll pick it right back up again later.