It's every hobbyist's pitfall that as they can't find the time or money to maintain their hobby, they are suddenly left with a lot of gear and nothing to do with it. This is especially true with people who own fish, and find that the feeding, cleaning and maintenance have become too much. However, there are things you can do with an empty aquarium.
If fish were getting too high-maintenance, or the species of choice has changed, it only takes a little modification to switch from aquarium to terrarium. Depending on your choice of animal--frog, lizard, or even spiders--the glass case needs only the proper substrate for the animal to live on, plus food accessories and other necessary equipment. Your current aquarium equipment, like lights and a thermometer may still come in handy for this project.
Small aquariums work best for this operation. Fill the aquarium with the appropriate sand or dirt, add ants and food, and the little critters should do the rest of the work. However, make sure that the glass case is sealed on top to prevent the insects from escaping, but do poke little holes in the top for air.
The aquarium case works well for hothouse plants when you need a little heat and humidity. You can plant the plants in dirt or place potted specimens inside the aquarium case. Make sure that the case isn't in direct sun and heat all the time lest you cook your plants, and allow for adequate ventilation to prevent mold and fungi from taking over.
If animals aren't your thing, the empty glass box makes a handy display case for valuables that you would like to show off. Depending on the size, you can put all sort of things in there, from miniatures to rare insect specimens, valuable books and other items. Either lay the aquarium on its side for rear access or have the opening face down, in which case you'd have to lift the entire case to access your valuables. You can use the tank right-side up with the lights if you want to light your display. Smaller tanks work best for this operation.
It's a little fancy but if you weren't going to use it any other way, your empty aquarium tank can make a good storage case. The benefit is that you can see what's inside. However, if using it to store items in a shed or other storage area, be sure not to stack things on top of it lest it break. Also, don't stack it in a high place where it might fall and shatter.
If you're feeling artsy, paint the the glass, and then use a low-wattage bulb or a cooler bulb like an LED to light it from inside. It can be a simple yet beautiful lamp.