Riding on the City to Farm ride ... no cars anywhere
I'll recommend two possibilities from the Boise Road Cycling Guide:
1. Cartwright Road Three Summits Ride (guaranteed great workout)
Distance: 17.7 miles
Recreation pace: 1.5-2 hours
Start: Junction of Hill Road and Bogus Basin Road
Go north on Bogus Basin Road to Cartwright Road. Go left.
Take Cartwright over two summits.
Mile 4.4 - Right on Cartwright
Mile 6.4 - Left on Dry Creek Road.
Mile 8.4 - Left on Seaman's Gulch Road.
Mile 12.2 - Left on Hill Road Parkway
Mile 12.6 - Left on Hill Road at Gary Lane
Mile 14.5 - Left on Hill at Castle
Mile 17.7 - Finish
2. City to Farm Loop (Scenic ride on the outskirts of Boise)
Distance: 16.2 miles
Recreation pace: 1 hour
Start: Overland & Five Mile Road. Park by Ken's Bicycle Warehouse in the 5-Mile Plaza.
Go south on Five Mile
Mile 3 - Right on Lake Hazel
Mile 4 - Left on Cloverdale
Mile 5 - Right on Columbia
Mile 6 - Left on Eagle Road
Mile 6.5 - Right on Hubbard
Mile 8.1 - Right on Locust Grove
Mile 10.1 - Right on Lake Hazel
Mile 13.1 - Left on Five Mile
Mile 16.2 - Finish
Be sure to dress in layers. The temperature might vary 10 degrees during your ride, depending on where you start. Bring an outer shell, which can be shed in the heat of the afternoon.