Just thought I would share a few ideas with you on cheap but functional, unique looking aquarium decorations.
Coconut Caves - Less Than $2.00 / 3 Hours To Complete
Flower Pots - 4" Diameter Flower Pot @ $.50, 6" Diameter Flower Pot @ $.97 / 30 Seconds To Modify, Can Be Put In Tank Immediately


PVC Pipe - 3 to 1 PVC Pipe Connection @ $2.96, 2 to 1 PVC Pipe Connection @ $2.50 / Can Be Put In Tank Immediately


I was able to arrange a few rocks that I had laying around the house to create a VERY sturdy cave for my fish. Nothing is worse than your rockwork collapsing on your fish. The PVC Pipe really helped support the rock. If you wanted to, you could cover it in your substrate using silicone. This would make it blend in with the aquarium and would give you a more natural look.
Custom Built Stand -
I built my stand to hold my 40 gallon tank as well as hold a 10 gallon underneath. I purchased 1 2x4x8 and 2 2x4x12 and a small piece of 3/4 MDF Plywood. It took approximately 3 hours to complete, and the finished product is working great! One day I am going to either stain it or paint it, but for the time being, it's doing exactly what I build it for. I left an overhang with the MDF on top so that if I wanted, I could always add sides to the stand. I could turn it into a cabinet. Always leave room for future modification. :D
All in all it cost me less than $25.00 to build this stand. Much cheaper than what you could purchase in a store, let alone if you could find one for an odd tank size.

Feel free to share other ideas that may have worked for you!