You can put soil all over. Most sands are not particularly safe for geckos, especially young ones. In the wild, they live on what is mostly a hard-pack, rocky area...so go ahead and do all soil (with rocks...or better, clay balls, beneath for drainage), if you want. As far as plants go, most are just fine (the leopard geckos will not eat them), but you probably want ones adapted to a dry environment, as you do not want to be watering them all the time.
G. A. Christian Bilou, Herpetologist
Founder/Director, Reptile Rescue Alberta
Past-President, Calgary Aquarium Society