Detolf! If you have £40. If you have £60, detolf and two lack tables to put it on
https://www.google.co.uk/search?q=ha...ixDQYQ_AUIBigB
Other than that - I would just look at things and see what inspiration you have! Some ceramic things can make good little hideouts or sand baths.
The book holders for shelves you mentioned are Knuff magazine rack things. They come in a pack of two I think. There's a little finger hole in the end that needs blocking off really so hammies don't get stuck in it.
Little plantpots can be good on their side for a hide-out and plant pot terracotta bases can be good for under a water bottle to help keep nails short.