Mesmerizing little thing I ended up with while getting a feel for using Microsoft's XNA Framework.
There are blue critters and red critters. The blue critters live in the mountaintops, and only descend into the valleys in order to mate with the red critters. The red critters have the reverse behavior.
On initially starting, a random terrain is presented with a white square in the lower lefthand corner (that's you). typing 'v' will randomly place one critter at a time. Typing 'p' will place 100 random critters. If a critter runs over your square, you will start riding the critter. You can use the arrow keys to move from one critter to an adjacent critter. You might try making it from one corner to the opposite corner, if you want to have a goal.
The terrain will gradually smooth over time. This can be sped up by pressing the 's' key.
Pressing the space bar will randomize the terrain, leaving any critters on a very jagged landscape. You'll need to use 's' in order to smooth this out.
The Escape key remove all the critters and randomize the terrain.
'+' and '-' speed up and slow down the critters respectively.