Balloon is the most common word for "气球" in English. But there's more to it than you think!
Fun expressions with balloon:
1. Hot air balloon - 热气球 (literally "hot air balloon")
2. Water balloon - 水气球 (for water fights, you know)
3. Balloon animal - 气球动物 (those cute twisted shapes)
4. When the balloon goes up - 当事情搞砸时 (British slang)
5. Balloon payment - 气球付款 (finance term)