Just inside the main entrance to the historical park is the Ramkamhaeng branch of the National Museum. The buildings house some of the small original sculptures found during excavations of the city, as well as reproductions of several of the larger artifacts. The museum now includes several buildings as well as a sculpture garden.
Since the museum opens after the park does, it's best to save a visit until later in the day. Although the air conditioning is usually off, the buildings still make a cool retreat from the heat outside.
The museum is open from 9:00 a.m. to 4:00 p.m. Admission is 150 Baht (3.90 USD). Admission to the museum is not included in the admission fee for the historical park.