As a US passport holder, you do not need to acquire a visa before entering the country. You will receive a tourist visa on arrival when going through immigration at the airport. Thailand has recently updated their visa policies, and you will now receive a 60 day visa on arrival.
While December is a cooler month of the year for Thailand, it will still be humid. Your body will not acclimate in time to get fully used to the climate, so dress light. Suggest wearing athletic, moisture wicking type clothing. Hybrid material board shorts and pants are great. Thin and breezy dresses or skirts as well. Just stay away from wearing cotton. You will sweat a lot, and cotton will stay soaked in this weather.
Strongly suggest you download the Grab app and setup an account. It’s like Uber for Thailand. Use it to hail a taxi and go anywhere you want to go. For ease of use add a credit card to your account. Grab is a “universal” app, so it does many things beyond just calling taxis. It also does food delivery, package courier, or even hire chauffeur services.
If you’re bringing foreign currency in for exchange. Search for a SuperRich location and exchange there. You will receive a better rate for bringing in crisp $100 bills of the latest design. If you need to grab cash quickly from an ATM. Look for a Bangkok Bank location, theirs machines are denoted by their dark blue color and diamond flower logo. Their ATMs offer the best exchange rate.
Under current laws, alcohol can only legally be sold during certain hours of the day. You can purchase alcohol between the hours of 11AM-2PM and 5PM-12AM. Keep this in mind when trying to purchase alcohol at a convenience store or elsewhere. Bars and restaurants usually skirt around this law in various ways. Just know you may not always be able to get a drink when you want.