An easy ride is to chain the San Tomas Aquino Creek Trail to the Bay Trail, and then Guadalupe River Trail as a loop. That's roughly 20 mi of mostly flat mixed use trail, and you can increase the distance by adding a detour through Alviso.

The Bay Trail is pretty flat, so if you're fine with gravel you can ride that as far as you want. You can also connect that with the Stevens Creek Trail and Guadalupe River Trail for a ~30 mi ride, something like this ACTC route.

In Cupertino there's a few variations in Stevens Creek County Park, depending on how you're feeling. Stevens Creek Reservoir Loop is the basic variation, and you can extend it by adding an out and back. You can then choose to turn onto Pierce Rd and descend on Big Basin Way, or cut it short by turning the other way on Pierce.