I use an Etrex 20 much as you described. I just googled "free GPS base maps" and found a bunch of websites with just that. I'll typically draw route on Google maps, get the KML file, then use kml2gpx.com to convert it into a gpx which I can put as a route on the GPS unit. If I'm strictly following a route, that's all I'd need, but of course the base map is handy to have too for improvising routes and such.
However, a GPS unit is not a substitute for a good paper map. Rather, it's a supplement that produces extra capabilities. If you're doing any kind of navigating or routefinding, you'll want something more usable than just the GPS.