Stevens Creek Trail for the one that's 5 minutes away. You're right, it's basically a green belt, but it's awfully handy for biking to work or just getting out. You can walk all the way down to the Bay, and then on to Palo Alto Baylands, for a close to 10-mile round trip.

20 minute driving time includes Rancho San Antonio, Stevens Creek County Park, Villa Montalvo, and Lexington Reservoir, all of which are perfectly adequate half-day hikes. Make it 30-40 and you also get the Los Trancos/Montebello/Russian Ridge area, Castle Rock, Almaden, Calero, Garin/Dry Creek, and Don Edwards.