Lots of things. Natural gas. Hydrogen gas. Unfortunately, they also explode or burn. There will be a party supply place that has a balloon fill tank. Welding supply places will have hellium tanks for rent with balloon inflator outlets. Just make sure you get good balloons or the He will quickly leak out. Also worked for jokers in the 60's They used Hydrogen gas in balloons and suspended Aluminum tins below the balloons and a chemical that would ignite a burning mixture. When viewed at night, caused UFO sightings to be recorded and then the fires blew up the gas in the balloons.
Chemist. Got stuck inflating about 200 balloons the night before a presentation. Presentator got cheap balloons and I had to re-inflate all 200 a few minutes before the show.