My clowns don't have any corals that have caught their eye, so they host a patch of hair algae. Before that, it was a powerhead.
Anemones definitely need a mature tank. 8 months to a year is the generally suggested minimum age for a bubble tip, though it can potentially be longer than that if the tank is started from dry rock. Rock flower nems can go in a slightly younger tank if it's doing well (if you have LPS happy and growing, you can probably support rock flower nems), but do still need stability.
If you want to try something hardy that your clowns might like to host, a patch of GSP (green star polyps), isolated on their own rock on the sandbed so they can't spread everywhere, may appeal to them. They especially like long, wavy things that ripple in the water flow. Rock flower and carpet anemones don't really do that, so don't tend to catch their eye too much, though you do occasionally get weird clownfish that will try.