Nice. What I used to do with flip books is draw some basic lines fairly hard, leaving an impression on the next page. Then I used the impression as a guide and moved the lines a bit each page.
The animations I found the most easy and fun were:
Stick figure diver. You can make him walk the diving board with trepidation, then jump up, hit the board (and bend it cartoonishly by doing a lot of frames before it springs back) and then do flips in the air and such. That was my first animation.
Hang glider. You can basically draw it like a paper airplane, and have it do lots of neat tricks. This was my second animation and it taught me to use size to make it look like it was coming nearer or going further away.
Car driving down a street. I drew a fairly detailed car from the front, and then drew sidewalks and buildings on either side and animated them going past. Then I added details like people going by, people throwing stuff at the car, jumping from buildings onto the car (hey, they were ninjas and it made a cool chase scene) and such. This was the first time I made a lot of different things move separately, and I animated them in one at a time.
The limitations of the flip books always frustrated me, and I had always wanted to try a medium that let me do more details and let it run for longer (the little notepads I used never let it go longer than half a minute or so) so I'm glad you asked this, it made me look into it and I may start toying with animation again.