Ballet,
My daughter tried ballet when she was 2 years old. She spent 30% of the time sitting on the side watching the other kids do their thing. She just wasn't interested so we didn't register her for the next class. She did look adorable in the ballet outfit and homemade tutu. At my daughter's request we tried ballet again when she was 3. Again there were periods when she refused to participate. As long as there is music running and jumping she's OK but as soon as some of the finer points of the dance are practiced she prefers sit on the side. Of course on parents day when I saw her perform with her peers, for the first time (but definitely not the last), I got a lump in my throat thinking how grownup she's become. Able to perform a rehearsed show was very touching.
Swimming
We've had the kids in swimming since the beginning. At 4 my daughter can swim the width of the pool. She loves the freedom that goggles give her in exploring underwater. I love the fact that I don't have to get into the pool with her. She swims with fearlessness and enthusiasm. I'm very proud of her.
At almost 2 my son loves the water. The cold pool doesn't phase him at all. Highlights of the lesson are throwing the toy and then kicking his feet to get it (then repeat). He isn't quite as fearless as my daughter was in the pool but enjoys it just the same. I can't wait until he can do this by himself as well.
Skiing
The grandparents gave skiing lessons to my daughter this past winter and volunteered to drive her to the hill most weekends. I thought skiing at 3 was a bit young but sure enough by the end of the lessons she managed to negotiate the biggest of green hills with a slow and steady snow plow. Some days I'm amazed by what she can do.