You can look at the ramps and steps gallery section for ideas. I think I'm going to use fiddlesticks like in this picture: (broken link removed) but I'm not completely sure yet. You can also bend a couple grids into a flat u ish shape so there is room to walk and there are walls. You have to cover the area where they walk in something, such as fleece, so they don't fall through and so they can get a good grip.