First, make sure you can use your miles during that time--I know I've encountered blackout dates when trying to use mine.
Second--you need to find destinations serviced by both United and American, the more frequently the better!
Most importantly, you need to figure out where you can get flights. Three months out during heavy vacation season itsn't much when you're trying to use mileage awards. You'll have better luck with destinations that aren't huge spring break/family destinations (Albuquerque to Taos/Santa FE?) or places with multiple airports (LAX/Burbank/Reagan).
Your vacation may need to be selected based on where you CAN go if you're set on using those miles.
"If you can't be a good example, then you'll have to be a horrible warning." Catherine Aird