You can see my Robo lay out in the 2008 'show your cage' (sorry Im 2 lazy to open Photobucket
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Mine pretty much zoom up & over everything. They particuarly like the wooden 'Barrel' from 'The Range', which they now use instead of a ladder to climb up a level. Sometimes I like to have the base of the tank relatively empty & sometimes I put things in - I have found they venture out more with the toys so that they can hide if big scary me comes along & makes them jump
Another great 'toy' is drift wood - again from the range & for only £1.99, or you get it from an aquatic shop. I originally brought it for my fish tank but then I tried it with the Robos & the fish never got it! But I did buy them some more! It depends if you want it to look natural (which is my preference at the moment). I like the stick bridges you get, sometimes they sit on top of them, sometimes they drag their millet in it & sometimes they sleep in there & sometimes they use it as a ladder!
ETA: They also love a good dust bath - & I love watching them roll about