I'm not all too great with plant species either, but I found this:

Plants of South America

It's not specifically northern, but you could poke around and get to know some of the species you like, and see if (a) they're available, and (b) they'll fit the dimensions of your tank.

Some of the hiding could simply be adjusting to a new home. I usually try to up the humidity, though, when I see a tree frog burrow into moss. Was there any difference after you got the humidity above 60?