Me and my friend stopped over in Lop Buri only for an hour, which was a good amount of time to spend there, but we wished we stayed at least the night. (We had to get the train back to BKK) The monkey's are hanging out mainly at a temple which is 5 minutes away from the train station. It's free to see them but it's like $1 for food if you want to feed them. They are crazy! They are seriously intimidating- they scared the hell out of me. One of them even jumped onto my backpack so be careful of your belongings!! They were also feeding them around 6pm so that's cool to see. They have little swing sets for them to play on to- this town loves their monkeys! Also there is plenty of accommodation and places to eat so I recommend spending at least a day there.