Well I'm guessing you'd want something easy to care for so you can still focus on your high tech tank the most... Hmm... I'd go with a corn snake
I know they are common so I'm not sure if that's a plus or minus for you but I always thought they were beautiful and interesting. Plus they will climb and use the floor space.
If your looking for something with more legs XD I would go with a leopard gecko as I like their coloration better, however I do know they are clumsy climbers so maybe a 40 high is a little much for them lol unless you would want to create easy to climb platforms.
Or if you really want more legs you could go with a tarantula. If you get an arboreal species I think they would be cool to watch as they would use more of the tank. Plus you could split the tank in half or in threes and the tarantulas would still have enough space. These are the things I would consider for an easy to care for tank.