There are some nice sightseeing spots around the city, like the big Buddha in the mountains. They have a few night markets with excellent and good priced food.
But if you dont bring your own vehicle, there is basically not much to see, except the monkeys. Having a car, we enjoyed it, but its nothing to go crazy about.
Lopburi is famous for it's monkeys which roam freely around the old town. There are abundant temples and old ruins.
Festivals are celebrated with a passion. Just outside of town is a lake with dozens of lakeside restaurants, beautiful sunflower fields (December) and nearby is a cave from which tens of thousands of bats pour our at around 6:00 pm. A definite "must see"! Well worth make a visit, even if you only have a couple of days to spend there. Consider renting a motor scooter so you can easily get out of town to the temples, lake, etc.