You might find the nightlife in Lopburi fairly quiet for a town of its size but there are a selection of places to catch a drink in the evening. Old Town has a few curbside bars, which are excellent for those who are still new to Thailand, as there are usually some foreigners about. There is also a small club look for the large "Ben More" sign next to a local park near the train station in the Old Town, but it is a little pricier than average.
The center of town has a variety of places, from hole in the wall local dives, to "The Bank", a disco that is frequented by Lopburi's young crowd not recommended unless you know your way around well; foreigners are rare in the Bank. Uptown has few drinking establishments on the main road, but there are a variety of karaoke bars and such down the back roads. Some of these out-of-the-way places are decent for a drink and some offer female company also not recommended for the newcomers.
Butterfly Bar
Nice little street side bar with beer, whiskey and food. Right across the street from the Narai Palace east gate. Gung and Steve are great hosts and the bar stays open until there is no one remaining. Side note, for farangs, there are usually a few 'westerners' hanging around. Enjoy.