Isometric Fist: Simply make a fist and clench it tightly for one second, and then splay your fingers out, straightening each one as far as it can. Repeat.
Isometric Pinch: Pinch your index finger and thumb together tightly for one second, and then spread them out as far as they can go. Repeat for the other fingers.
Finger Extensions: Make a fist and cover the fist with your other hand, slightly cupped, as in the salute used by some kung fu or kenpo schools. Extend your fingers outward from the fist, and apply resistance with the covering hand. Do the same action for the thumb. This isn't an isometric exercise (an exercise where you tense a muscle but keep the limb stationary)--you do want to go through your fingers' range of motion, just under resistance from the other hand.
Fingertip Pushups: Just like regular and other types of pushups, except that instead of starting by holding yourself with your palms flat on the floor, you hold yourself up on your fingertips. If these are too hard to start with, do fingertip pushups against the wall. Bruce Lee used to show off by doing these by supporting himself on just the index finger and thumb on one hand. You might want to start out using all ten fingers, though.