Wheel Fun Rentals - $11 per hour or $41 for the day. Located on Cabrillo across from the wharf. This option is great for exploring nearby bike paths and downtown spots. They also offer fun local tours.
SB Fitness Tours - Rentals for $75 per day. You can have your bike delivered to where you need it (for a little extra cost), which is great for checking out the local trails. If you're new to the area or the sport, they also offer guided tours in the mountains.
SB Bikes To Go - Rentals are $85 per day and if you have a car, you can rent a bike rack for $10. They also offer FREE delivery and pickup anywhere in SB, Goleta, Montecito, Summerland, and Carpinteria. Guided tours for all different levels are available as well.
More maps for do-it-yourself rides here.
Here are some to get you started. Check out more on the MTB site
Elwood Mesa - 5.3 mi (8.5km)
Lower Oso - 1.1 mi (1.7km)
Elings Park - 5 mi (8km)
San Ysidro - 4 mi (6.4km)
Knapps Castle - 14 mi (22.5km)
West Fork - 2 mi (3.2km)
Buckhorn Loop - 34 mi (55km)
Camuesa Connector - 10 mi (16km)
Jesusita - 7 mi (11.2km)
Little Pine Mountain - 17 mi (27.3km)
Romero - 6 mi (9.6km)
Gibraltar/Painted Cave Loop - 30 mi (48km)
Mountain Drive - 25.6 mi (41km)
Tunnel Trail - 5 mi (8km)