You really cant go wrong with Java moss and fern. Most Anubias are easy to keep as well, but check the type and conditions. Four leaf clover is a great aquatic plant thats next to impossible to kill and spreads by runners so can form a nice green carpet. Water milfoil is another plant which is very hardy(can live just floating on the surface), tolerates poor water, is a good oxygenator and makes a great place for fish or amphibians to play or hide.
I like Hygrophillia Lacustris for tanks with land and water as it will happily grow above the water line and is quite hardy.