There are a bunch of free topo maps on the internet - I use those. Google search will get you to a few sites that host them.
I downloaded those topo maps first to my GPS and then added the trail map overlays that I described above. You can then turn the maps on/off if you need to.
The next step that I have not done yet is to download the sattelite image files for my riding areas. That requires a 1 year subscription for $30 to Garmin - I have not pulled the trigger on that yet, not sure if I really need it or not. I don't use the GPS for much - just to check where the trail is if we are trying a new route and I use it to check wilderness area boundaries.