You could play 3D videogames and become very good at them. Or you could suture and tie knots, many different kinds of knots and surgical ones. I'm sure you can find figures online.

But while these simple techniques may be helpful in improving your dexterity, nothing will come close to preparing you for what you'll learn in the operating room as a surgical resident. And go easy on the knot tying because you don't want to develop too many bad habits which will be hard to break in the future, just as it's not a good idea to learn a musical instrument without lessons.

You might also want to go here and watch a couple of live or pre-recorded surgeries:

http://www.or-live.com/