With a US passport you will receive a free 30-day visitor visa on arrival. No fuss, no fees, even for a short transit. Difficult to say whether immigration will be busy at that time.

Even if you can get your bags checked through, it's worth leaving the airport for a few hours to see something of the city and/or grab dinner. If you have to collect your bags, there are storage facilities landside at the airport.

Just make sure to get back in plenty of time for security and pre-clearance.