Sand or gravel is ideal for plants
Especially if it's not too coarse and not too fine. If the gravel has too big rocks, it's hard to make the plants stay where you want them for longer periods as the fish can easier dig them out.
So if you have gravel that's about pea size (which is ideal for goldfish too!) you can easily put a few plants there! Make sure that you buy hardy plants with strong, waxy leaves. Such as different kinds from the Anubias family
Or Java Fern, which tastes bitter, so the fish don't want to eat it. And if you still want some with finer leaves, get some hornwort or elodea and feed the fish with some vegetarian food from time to time to keep them from eating the plants too fast. Something like algae wafers (Hikari) or some sea weed (omega one).
You easily place the plants where you want them before
you fill the tank completely. Just put water there until it'll cover your biggest plant and start placing them one after another where you want them. When you've decided that, you can just dig little "craters" under each plant and put them in, after which you fill in the hollow with gravel, and you should be ready to go!
If you have goldfish though, it could be good to weigh the plants down with some big pebbles or something