I use two apps for this. There are probably better apps, but I can only speak to what I have, and these are pretty good for what they do.
Topo Maps by Philip Endcott allows you to download topographical maps for offline use. You only download the ones you want, in in high or low resolution. You can place waypoints and do other basic navigation functions.
Motion X GPS HD allows you to catch up to 2 GB (depending on settings) of map data, so if you look over your destination before leaving the internet your good to go. It offers several map sources including terrain maps from MotionX and Bing. The Motion X terrain map is decent, though not as detailed as the maps you can download for the Topo Map app, but there are way more bells and whistles. Of course that means more to learn.
Neither app has an outstanding UI. They take a bit of fiddling and reading of help files to figure out.
I'll include a couple of screen shots of each map shortly. Note that the Topo Maps app uses a national repository of some kind, so the maps themselves vary from region to region, and they may not be as up-to-date as some online map sources.