Unrealistically-
The power to manipulate time.
Slightly less unrealistically-
Speak another language fluently. I speak English & "American Medicine" (as my friend calls it) fluently, know Hebrew decently, and enough French to get around. I'd like to learn German.
More realistically-
Learn to play Tennis- one of the few sports I've never even tried. Growing up my main sports played soccer, lacrosse, and swam and sailed competitively. I've played all general sports (baseball, football (non-competatively), basketball, golf, badminton, ultimate frisbee, etc. somewhere along the way. I have even tried my hand at more exotic sports like crew, curling, water polo, and real polo (2x). For some reason tennis never made its way into my life.