A day pack to carry towels, change of clothes, etc comes in handy. River tours are comfortable with water shoes and rash gaurds (but t-shirts and sneakers that you dont mind getting wet are just fine). Out on the cayes a large brimmed hat, sunshades, sunblock are vacation-savers. In the jungle, lightweight long pants, and long sleeve shirts over tank tops are comfortable. On some jungle tours, knee high or short pants are fine.
Currency: everyone in Belize accepts US$ dollars. Most major hotels, tour shops and stores accept credit card – but on-site and remote gift shops and vendors do not have credit card facilities – a little extra cash comes in handy.