Topo maps should work fine but you do have to turn off the City Nav maps to see them. Topo will not auto route on the GPS unit, but routes created on a PC can be uploaded and followed. IIRC you can create a route on the GPS (auto route) in City Nav, save the route, then turn off City Nav with the route still active and the SP will follow the route in Topo. This works on my Chartplotters and I'm pretty sure the Zumo users over at ADVRiders have used the process so it should work on a Street Pilot. All of this is possible on a Mac using RoadTrip, MapManager and MapInstall. The Topo maps will have to be installed on a PC first and converted to Mac format using the converter available from Garmin's website.
Try one of the free Topo maps available Here
to test it on your unit. There are also some good tutorials on the site. Good Luck.