You would be surprised how many items you could pick up from your local hardware store. I recommend any of the vivarium "build" threads on this forum to see ideas/concepts. You may not be able to get too much in the way of land critters if you want most of the emphasis on the water, but this can be okay.
I would try something like this (I am at work so I am just thinking of very rough idea off the top of my head). Assuming it is a 20gal high, what if you got some sort of barrier(glass or acrylic) and siliconed it into your tank, leaving 1/3 of the tank on on one side and 2/3 on the other? The barrier would be maybe 12" tall, Then, you could use foam (like Great Stuff) on the water side to create some sort of "stream bank." You could coat the foam with sand or a dark concrete mixture. The land side would have a false bottom of a few inches using plastic eggcrate. The rest of your background(above the water) could be made with malaysian driftwood/cypress and Great stuff covered in more of that concrete mix or coconut. You could create holes where you could plant some flora.
I dunno, thats my very rough idea.