Have you ever wanted to make a retaining wall like the ones you see alongside the highway/? You know the ones – they have reliefs of mountain ranges, trees, buildings, animals such as bears, dinosaurs, Buffalo, etc. The only problem I can see is that the concrete will stick to the wood. If I ever get around to it I would use a router for my reliefs or probably cut them out of plywood and screw them on to the original form base. Then I would seal the plywood with some good paint or probably use fiberglass resin. I would use the “Gel coat” type – it has wax in it. If you are adventurous you can buy the gel coat without the wax pre mixed and add your own. I know fiberglass guys who do this (they work on boats not molds). They claim it is better – you can build up a couple of coats and then add wax for the final coat. This way you get a stronger layer of protection. The thing here is if you use the wax type you can’t paint more gel coat on top – it won’t stick. Make sure that there are no undercuts in your forms and that they are as smooth as you can make them.
Paint on some gel coat mixed according to the instructions, let it cure and you should be ready to go. I think I would also use a spray on non-stick like “Pam” as extra insurance. I would test this before I went to my project because there is a lot of pressure on the form when it is full of concrete. Be sure you make your form as rigid as you can. Use 2×4 bracing on the outside. If it flexes your gel coat may crack and then you will have concrete sticking and of course a ruined casting. The critical thing here is preventing the concrete sticking to your form. Do all you can to stop this and you will build a great looking wall.