If you're trying to kill time or are into temples, then maybe I would suggest it. But if you have limited time, then I would pass and go to kanchanaburi instead
#2 heavanse has been a member since 2/3/2010. Posts: 11
If you're only here for a day or a couple of hours, then I would say yes. I stopped here to kill time. I've been here 2+ days, taking an overnight train to Chiang Mai at 8, and I can't wait to leave. I'm sure it would be different if I was traveling with someone, but it's a small town and not much to do.
#4 heavanse has been a member since 2/3/2010. Posts: 11
Lopburi is okay. The museum there is quite good, and the old market area is interesting. Of course, if you want to see monkeys then you'll get your fill of them around town and at Wat Prang Sam Yot.
The countryside around Lopburi is beautiful and it would be worth it to rent scooters and go out exploring, especially around Ban Lum Khao (I used to live there) and Ban Nong Sai Khao. They're lovely little villages surrounded on all sides by rice paddies and rolling hills.
Taking in Lopburi and Ayuthaya in one day-trip, though, might be pushing it a bit. Ayuthaya, alone, has enough to keep one busy for most of a day, and even night as many of the nice old temples are illuminated (though I can't speak for the place since the flooding).
If it's temples that you're interested in, I'd recommend making that stopover on the way to Chiangmai in Sukhothai and Si Satchanalai instead. Sukhothai has far better temples and lots of good and inexpensive places to stay. With scooters you can see both the Sukhothai and Si Satchanalai historical parks in one day. Si Satchanalai is a fantastic place that few people go to. Also, from Sukhothai (Phitsanulok, actually) there's no need for a sleeper train to Chiangmai.
Lop Buri is easily worth a stopover for a few hours on the way to Chiang Mai. As Tilapia says, the museum is quite good (albeit a bit expensive), and there are plenty of other interesting ruins to visit that will keep you entertained for a few hours.
As much as I like Lop Buri, I think Ayutthaya is a better option for a stopover on the way to Chiang Mai. There is more to see in Ayutthaya in terms of museums and ruins.
I also agree that you'll be much better off picking one or the other, as trying to visit both in one day is simply too much. If you only have a few hours, Lop Buri is good, but if you have a half day or more to kill before catching the overnight train to Chiang Mai, I'd pick Ayutthaya.
Have fun. Regards.