Another idea for plants are carnivourus plants. Years ago I put together a nice terrarium with them, you can find some nice small specimins online.
My best suggestion though is to think TALL. Plants grow up and cane outgrow an aquarium pretty easily.
The carnivourus set-up I had was only 12" wide by 12" deep but was 24" tall, which made it perfect for blooming plants.
