I'm all for original and unconventional tank decorations! If you add rocks etc to your aquarium, make absolutely sure they are safe. Many can be toxic, dissolve in water, or play havoc on water chemistry. A safe and pretty thing to add to an aquarium is quartz crystal. It makes an eye catching centerpeice and is perfectly inert. So is lava rock, just be careful because it is sharp. Round river rocks are also great and make a totally different feel than the more jagged ones. You can pick them off the land, but test them first: add a couple drops of vinegar to the rock. If it fizzes at all, don't use it
I also like to use silk flowers from craft stores and I'm redecorating my female betta's tank with those. Just be sure to cover any metal wires with aquarium sealant. Metal in the tank can be toxic to fish.
However, I do like to buy some store-bought decorations. I bought a lovely peice of resin driftwood and it really helps my tank stand out. They are coming out with some really neat and realistic peices, and the selection is definately improving over the years.