Stay at one of their partner hotels or rent one of their partner car rentals, and chose their "bonus" miles.

Shop through their mileage mall and get something inexpensive. An example is buy at Circuit City a package of batteries. With in-store pickup, there will be no shipping charge. So for $3, you may get 9 bonus miles - which will keep your account active for another 3 years! (All 10,000 or 150,000 miles!)