Sorry, ke... misunderstood on first reading. It sounds like you might have to spend 2 nights. Your best bet is to rent a car or check the bus schedules and see when the express buses are running to/from Belize City and SI. When I was looking to bus it to Guatemala I know there was an express bus there that left from around the swing bridge at 9:30am. It was $25US and went through SI, I believe. Give S&L Travel and Tours a call and they should be able to give you info on that bus (501 227-7593). Alternatively, when I traveled back to BC to catch my flight on my last day, I took one of the Novelos express buses. It only cost $11US one way and the times varied... 5:15am, 5:45am, 10:15am, 11:15am, etc... One of those buses could get you back pretty early after your second day. The excursion I did left around 9am and didn't get back until well after 4pm so it sounds like it would be too late for you to get back to BC/AC after that. Staying an extra night and then leaving first thing the next morning sounds like it might be the only way.