Walk along the pier or run along the beach trail that follows the coastline! We (and Bowie) like to do this run whenever we're back in town! You can also rent a board and a wetsuit at stores by the pier if you want to surf!
Grab a beer at Artifex on Del Mar, a glass of wine at Rancho Capistrano Winery, or the best Mexican food at South of NIck's! Downtown San Clemente has tons of great food and shopping options.