You can use absolutely anything and they will try to jump up on it, you can stack a taller box next to a smaller one so they can use the smaller one as a step. I started by collecting lots of those cardboard fruit trays that super markets give away. My local Tesco leaves them at the end of counters for people to put their shopping in.
Once I stacked up about four of them and put the bottom two open sides together and cut a little entrance hole so the bunnies could use it as a hidey hole, then I stacked two boxes next to the four box tower and they used it as a step up. They are great because you can keep changing the layout which keeps them from getting bored.
Other ideas are:
Childrens stools or chairs
Step up stools (about £5)
Plastic storage boxes
Up turned plant pots
Open box with open side up so they can jump in.
Garden chairs
My rabbit knows no bounds when it comes to heights, he will attempt anything ... even the garden wall :shock:, which he has managed to get over into next doors garden! He can even take a running jump and land up on something nearly a metre high. :roll:
I hope this helps, and is the kind of ideas you were looking for.
You really don't have to spend, just look around what you have lying around at home and in the garden.
Kate