Get a bunch of java fern. Get a rock or piece of driftwood or other tank decoration. Superglue the rhizome of the plant to the decoration of your choice. Put the decoration into the tank.
Or get a plant weight and tie wrap the rhizome to the weight and put the weight into the tank. Hide the weight behind a piece of rock or decoration.
Java fern is a rhizome plant and doesn't need to be planted in the substrate. Just weight it down so it doesnt float around. Anubias is a similar plant.
If you stick to low demand "easy" plants, gravel or sand is a perfectly acceptable substrate even for rooted plants. You can use root tabs if you need to provide some extra nutrients.
These low demand plants dont need much of anything special. Just a light really and if they are a rooted plant some substrate. I keep saying i think i could keep java fern alive in a bucket of water in the shed. Its bullet proof.