Most stem plants like hornwort or anachris are very easy and fast growing. You put them in water with a bit of circulation, a little light and some fish and the next week you have twice as much. A planted type that grows well for me is Cryptocoryne aponogetifolia. Beware that the crypt gets very tall and will turn back on itself in any but the largest tanks. Slower growing but easy plants are java fern, Anubias nana and java moss. I think java moss will even grow with just the light that gets into the fish room. I have a clump in each of two unlighted tanks as cover for some livebearers and it is still looking nice and green after about a month.