I had the idea to turn an old gumball machine into a small, fun fish bowl. Here's my process.

You will need:

Various assorted screwdrivers
Drill and bits
X-acto knife
Sandpaper or sanding block
Hacksaw
A gumball machine on a stand. Do not try this with square gumball machines. They must be round, preferably glass.
Glass and glass cutters (or the phone number of a glass shop)
Silicone aquarium sealant (must be aquarium sealant, as bath sealant is not as strong and may contain chemicals that will harm your fish)
Rust-proofing spray paint
Push-in rubber grommets
Nuts and bolts and washers as needed
Some scrap aluminum
Gravel
LED Betta light
Small aquarium filter (I used a Penn Plax Smallworld Filter)
3-plug extension cord with grounded prong
Rubbing alcohol

Step 1: Disassemble and Clean

I got my gumball machine off of Craigslist. It had some really gross stale gumballs in it when I started.

Take the whole thing apart carefully, and clean everything really well. If paint is cracked or chipped, don't be afraid to sand it off and repaint it. I didn't need to. I did remove the crumbling 10c sticker, even though it would have looked nice. There was a cork ring between the steel on the top of the globe which was stuck to the glass. I scraped it off and sanded the edges of the glass down. If the steel parts on the bottom of the machine are stuck on, try to get them off. They've likely just stuck from age and pressure, and aren't secured.

Clean the entire inside of the globe with rubbing alcohol and let it dry out.
At this point I also took the stand outside to sand and paint the bottoms of the legs, which were in bad shape.

Step 2: Setting Up Your Base

Drill a hole near the bottom of the base, or where the bottom meets the sides of the case, for your power cord. Then, pick a spot in the back to drill for your light and filter cords. I went a little above dead center. Rubber grommets (~$1.50 from Home Depot) will go in these holes later to prevent wear from the metal.

Arrange your power cord, filter, and light assemblies in the base. This takes some trial and error. I tied all of the loose cable together, and ended up strapping some foam to the bottom of the filter pump to keep it from rattling against the metal. It was helpful that my light's power cable was two pieces, making it only necessary for the holes to be wide enough to pass the connectors, and not the entire light.

I nestled the connectors for the light in the candy chute, to make it easy to turn it on and off without having a cable hanging outside.

Step 3: Finding a Way to Secure the Globe

In its service life, the gumball's globe was kept on the base by the rods running up through it. Since we're going to be sealing the bottom of the tank, a new way has to be found to keep it from falling off. To achieve this, I fabricated a bracket of sorts from the steel piece that was at the bottom, and some scrap aluminum. I spot welded a washer and nut over a hole I drilled on top of the one that goes all the way down. In the photo below is how it looked when I test fitted it. I drilled a hole through the bottom of the stand so I could run a threaded rod up through the base. Turn the two nuts, and the rod moves up to secure the bottom of the globe.

(I also flipped the bolts securing the machine to the stand, to make it easier to remove)

Step 4: Making the Globe Into a Bowl

Now, measure the width of the circular opening at the bottom of your bowl. Cut a disc of 1/4" glass, or cal your local glass place and have them do it for you. It shouldn't be terribly expensive. Make sure to have them grind the edges, not temper them.

Using your aquarium sealant, lay down a nice thick bead along the circumference of the glass disc. Lay it on to the bottom of the bowl and smooth out the sealant on the inside. Let it dry as instructed.

Once the disc is dried enough to hold (doesn't have to be fully cured at this point), repeat the process with the hold-down on the bottom and the rubber ring (if your tank has one) at the top.

Let this whole assembly dry to full cure (look at the package) before doing anything. I suggest doing this at the beginning, so you can make the most of the day or two it needs to dry.

Step 5: Set Up the Top

Flip the cover over and line the retaining ring up inside it, marking the front and back. Drill a hole in the back and run the light and filter cord and tubing through it. Rubber grommet here, too. Use some of the aquarium sealant to stick the light to the inside of the cover (it's a very strong adhesive), and then secure the retaining ring to the bottom of the cover.

Lastly, squirt some sealant into the lock hole and push the lock into it.

Let it all dry.

After some time, I realized that the bare metal would cause metal poisoning in the tank. To combat this, I disassembled the cover and painted it with rust-proof paint.

Step 6: Wait

Now came the hard part for me: waiting for all that sealant to cure.

In case you're wondering, I use the pill container to organize screws and small parts.

Step 7: Water Test

Now it's time to water test your globe. Never test it on the base, as if it leaks you'll kill your electronics. I did my first test on the concrete outside, and I'm glad I did. I found a small leak, which I fixed and let it dry again.

Step 8: Setup

Find a nice spot for your aquarium. Rinse your gravel and plants really well before you use them. I used a mix of rainbow and black. It looks terrible. Stick the filter to the inside of the tank, and hook everything up. Condition and add water. Mine holds about 6 liters.

Step 9: All Done!

Add a fish! Bettas are among the few types of fish that will thrive in an unheated tank like this. I chose red to go with the color of the machine, but that's up to you.

I quickly realized how terrible the gravel looked, and switched to all-black. To hide the tube and cord, I printed a gumball background.

EDIT: Later, I decided to spring for about two dozen glass gumballs, and dumped them in the tank for a good gumball look.