Easy Athletics/Acrobatics Skill Increase
All you need to do is find a place where you can heal and either jump around really quickly or swim and run around (or both at the same time).
The best way to do this is to find a ramp, preferably with a bed close by (the bandit place near P is great). Look at the ramp (from either the top or the base, it doesn't matter) and run up it (forwards or backwards). At the same time, jump as rapidly as possible. On the PC, hold or while pressing like a madman with your right hand. When you reach the top, jump or run back to the bottom and do it again.
The effect of this technique is to reduce your jumps to take a very minimal amount of time. If you press fast enough, you can get five to ten jumps per ramp climb. Do a few sets and then check your acrobatics skill, it will have increased tremendously (depending on what your current skill is of course).
While you're working on jumping, the motion of running up and down the ramp also helps increase your athletics skill. In about five to fifteen minutes it's possible to gain around six or seven skill increases.