The ancient city Sukhothai, or “the dawn of happiness”, lies about 320km south of Chiang Mai. Roughly a five-hour bus ride will get you to this very first capital of Siam (13th century). The restoration of the many ruins and sites of historical interest started in the 1950s, and in the early 1990s, the site was designated as a UNESCO World Heritage Site.
The historical park is just a little too large to explore on foot, and is best done by bicycle. Hire a bike for the day, and get to the park early before it gets too hot. Bring a picnic lunch. It’s very pleasant in the park, and surprisingly quiet too. It is green, with ponds and lakes, and it’s lovely to walk/bike around, take your time to enjoy the ruins, and sip an iced tea when it gets a little too toasty. If you have time, venture out further into the less visited parts of the park, and you’ll pretty much have the place all to yourself.