one of the easiest plants to grow is Water Sprite (Ceratopteris species). It is a floating plant but also does well when planted in the gravel. It does not need much light or fertiliser but will enjoy it if provided.
If you don't have a light on your tank you can see what the local hardware store has in the way of LED spotlights. There are relatively cheap and can provide ample light for Water Sprite and lots of other plants. And LED lights are pretty cheap to run and they don't need the globes replaced.