Terraria are easy so long as you keep to a few guidelines:
1. Use plant that like low light levels. Mosses and liverworts as Rowan and JingleJoe are doing, small ferns, african violets (you can get miniature growing ones of these) and some of the smaller carniverous plants - smaller sundews, butterworts and the terrestrial bladderworts are good.
2. Use plants that like very wet or swampy conditions. This includes most of the ones I mentioned above, but the african violets and the ferns like to be a little bit drier than the other ones.
3. Keep it cool. The enclosed glass can really heat up and roast your plants. Unless you go for cacti, in which case ignore the keep wet advice!
4. If you use a demijohn then ongoing maintenance is a problem because of the small opening, so I'd recommend going for plants that are really small (mosses, liverworts, bladderworts) rather than larger ones (violets).
Or keep going with the grasses - you're obviously in the right conditions!
I'll take some pics of mine tonight when I get home - I've got lichens in one small jar, sundews in a large container, plus the two ward tanks and the miniature orchid tank.