The miles and miles of sidewalks of Mission Bay Aquatic Park are perfect for Rollerblading and skating; you can admire the sailboats and kites while you get some exercise. Cheap Rentals Mission Beach (3689 Mission Blvd., Mission Beach, 92108. 858/488-9070 or 800/941-7761. www.cheap-rentals.com) has good prices for Rollerblades, especially for the full day; rental comes with helmet and knee and elbow pads. Holland's Bicycles (1201 1st Ave., Coronado, 92118. 619/435-7180) is the place to go to rent skates, blades, and bikes to cruise the beachwalk of Coronado. Skateworld (6907 Linda Vista Rd., Linda Vista, 92111. 858/560-9349. www.sandiegoskateworld.com) has several public sessions daily on weekends, with adults-only skating on Tuesday evenings. In addition to private group sessions for bladers and skaters they offer a really-and-truly women's roller derby (the season is February through May). If you want to learn to make legal roller derby hits, avoid penalties, and take a fall, join their Derby Dolls Boot Camp.