You can get a lot of really easy super cheap plants. The best for the buck are
Moss (any kind)
All those plants dont really need and care at all. Just time. They are all slow growers except the Jungle Val and the Rotala. They can make a super amazon looking tank. Heres mine for an example. I add co2 and stuff but its really not needed to grow these plants
The plant on the left is Java Fern. Its a Trident one but its super easy to take care of
The plant right in the middle of this one is Anubias. Slow Grower but really strong pretty leaves. And you can see the moss attached to my wood.
Knock yourself out! Plants are almost as fun as bettas
Cant make smaller pics??? Edit wont let me I guess