The temple was originally a monastery with a pavilion and a cave where monks came to practice dharma. The cave is still a popular place for that activity but, for a curious tourist, it is rather the whole upper part that is worth the detour. Between the gardens below and all the elements up to the top, it would take hours to appreciate every detail of the decoration of this temple whose construction still goes on.
I couldn't verify but it seems there are illuminations at night which are also worth a look.
Wat Simalai Songtham is a temple and when you visit a temple in Thailand there are some rules to know and follow :