I have 2 buns with 2 large dog kennels, which I leave open all the time (but have the option to lock them in if needed occasionally).
The cages are stacked vertically and the doors are propped open. They have no problem jumping up to the upper cage, which is waist height. I put scrap carpeting down in the cages and the laminate floor around the cage to make sure they had traction.
On the first day though, I had to pick them up and put them in there with treats, hay, toys, etc. Then once they understood all the good stuff awaiting them in their cage, they gathered the confidence and now jump up and down freely in the top cage and run in and out of the lower one too.
They love to jump and climb higher on things and explore. Not all bunnies are like that though. Some do need ramps.
Now when I want them to go to their cages, its easy. I just rattle their bag of hay or pellets or treats and they come running and jump right in. I make sure to deliver on my promise though and give them some whenever I rattle it. 