Here is a fun game, which is also good for servers.
(The cover picture, which I wanted to upload eig. was unfortunately too large!)
First, find a river course that you think is long enough. Then you build your start line and your finish line by building two "towers" (of any height) of fences at the same height, which you then connect with black and white wool.
Underneath, you put some boats.
Now, if you want, you can add a few more obstacles to the course of the river. Be very creative...
Here's an example:
Now the race is ready to start!