I'll be in Northern Thailand for 2.5-3 weeks, then I will visit other countries, then return to Thailand to visit the islands for 1.5-2 weeks.
I read I have 30 day visa - does this get 'reset' when I leave the country?
Yes, your visa will reset when you re-enter the country. When you arrive to Thailand by air, you get a free 30-day visa. Ignore any kiosks for temporary visas at the airport -- they don't apply to US citizens. You'll get your visa when your passport is stamped. If you enter Thailand by foot at a border-crossing, you get a free 15-day visa, so by either method of entry, you should be fine to stay for 1.5-2 weeks after you visit other counties. If you decide to stay longer, you'll have to exit and re-enter the country again, or go to an embassy and get an extension. Most people just do visa runs at different borders. Enjoy your trip! about 6 years ago