How about a miniature African violet or other gesneriad? Miniature leaf begonias are beautiful too. You need something that doesn't need/want direct sunlight to flower for obvious reasons. Check Logee's Greenhouse -- I haven't in years, but they used to have a miniature plant section in their printed catalog that are ideal for terrariums. Remember with flowers in a terrarium, you have to reach in and clip off faded flowers (long tweezers and long-handled and angled tiny scissors like those used for surgery are great tools) or they'll turn into brown mush -- in other words, don't make a bottle garden.