Moss balls are cool little plants to have in a tank, especially a small tank. They roll around, and some Bettas even play with them :)
If you can get some Water Wisteria, it grows pretty fast, and floats on the top unless you weigh it down. You can cut off stems of it once it grows and they will become seperate plants, so you'll have multiple starting from only one. If you have it floating on the top, she might enjoy resting under it, and if you weigh some down, she can sit on the little fronds. They're a pretty cute little plant too :)
Also, if you can get a 2-3 gallon critter keeper, they are pretty inexpensive tanks if you want to keep the red girl too. I have three 2 gallon critter totes for my three girls, and they were about $11 each I think.