The problem with plants in the leopard gecko tank is that the humidity goes up when they get watered. If you really want to do it, one idea would be to get some succulents that don't need much water (don't get cactus with sharp spines), put them in small, attractive pots and take them out to water them. The other problem is when you start to get into plants, you need good lights for them and many leopard geckos aren't comfortable with lights. You could always get fake plants, branches, interesting hides and things like that to make the tank look nice. If you like the less naturalistic look, some people have done wonders with legos.
Fake plants are the easiest option for you. Check out your local craft store (michaels, hobby lobby, etc) or even walmart! You have a lot of options for fake plants for pretty cheap