The best vacation day trip I’ve ever taken was to Ayutthaya, the ancient capital of Thailand that was destroyed by the Burmese in 1767. Today, most of the city is a UNESCO World Heritage Site, with tourists heading to the ruins of the temples and the Khmer-style architecture that remains.
It was a short train ride from Bangkok with unreserved seats, which meant that anyone traveling further along the line could force me stand (at least I had a seat on the way back). Ayutthaya would have been more enjoyable had I rented a bike or elephant. It also would have been worthwhile to spend the night in the quiet city.