I love an empty 20 gallon, that's the best of the best for small sized tanks. Of course, you say you're not looking for more animals, which limits my options (normally I'd totally suggest a couple particular species of gecko or frog-toad). It also happens that my fish tank is a 20-gallon, so I have a lot of stocking recs for that size tank if you were interested in the fish life.
Are you at all interested in plants? That's just about the perfect size for a little in-door gardening. All you need is to decide what you want to grow... My recommendations would be carnivorous plants, low-growing herbs like thyme and oregano, or maybe a succulent garden. Grab a little grow light - my rec would be a NICREW ClassicLED Aquarium Light with Extendable Brackets - grab your plants - bam! Perfect wee garden.
Feeder tanks are a stellar idea, either to feed your dragon in or to breed feeders in. That size tank is perfect for roaches, if you want.
If you're feeling real ambitious, it's not that hard to break down a tank, but then you have 5 pieces of glass and some trash instead. Still, there's a good bit of crafting you can do with glass, especially if you're confident enough to cut it yourself.