You might try to bump the ambient temps a few degrees.

Getting some climbing stuff in there is no guarantee that he won't climb on the thermometer anyhow. He may just like it.

That being said...if you want something that you know will be sturdy and won't fall over...why not check the fish department of the pet store...look for some of the big castles or whatever that they have for larger aquariums. Anything with a wide base and a low center of gravity should be perfectly safe. And it'd be visually interesting as well.