Would be cool to figure out bumblebees. You could get a nuc hive of honey bees and just let it die off or not let it grow bigger, I guess. They may swarm though, so, probably a little high risk for this particular set-up. A nuc can be had for $200-$300 and is pretty small, but will go get the job done. Just make sure you close the door to the GH when you have bees!
I am loving having a colony and it has become just as much a part of my garden as any other plant or maintenance job. Absolutely adore seeing them zoom around.