Ambulia, crypts and narrow leaf java fern would be good in a low tech tank, I have these and they work well.
You could put the ambulia at the back, crypts down the middle and/or front and the java fern on a rock or piece of wood as a feature. In a small tank the narrow leaf java looks more in proportion than ordinary java fern as the leaves are thinner. You'll need to trim and replace the ambulia from time to time but it will give you some height and hiding places. They'll also hide under the crypt leaves.