Fill it with air. Bet you can't fill it to the top ;) I need photographic evidence.
a dead body.
Water then some fish
How about fish
Octopi. Or an axolotl.
Keep some animals captive in an environment that is an impossibly small fraction of their natural habitat in terms of space and resources - for fun. Magnificent.
You could probably advertise to give a home to an aquarium type animal.
A Terrarium with lots of nice plants or an herb garden :)
erm 40 gallons of water? then some fish? tropical fish are great but you would need a heater and a water pump
plant cacti in it...there are tons of varieties and they don't require a lot of care.
Colombian boa, low maintenance pet
Eyelash crested geckos. Those things are awesome.
They're really easy to care for too because they have a plant based diet and everything.
Anything you put in a tank that size is going to be too messy and too pricey. I would trade it in for a 20 gallon and stick low maintenance fish in it.
Fill it with marbles, that would look nice.
We love leopard Geckos. Low maintenance...nice critters.