The last time I went to Nakhon Pathom, I went by train and I recommend it to those who like a different way of traveling. You can visit Nakhon Pathom in a day. Its principal point of interest is the chedi, a bell shaped dome which shelters a relic of Buddha, which is the highest in the world at a mere 127 meters tall!
But as I had already seen the chedi, even though that wouldn't stop me going again to admire the fervor of the Thais and the now finished renovations, this time I took the time to walk in the streets near the station and eat in a really local restaurant, to experience what I call the "real" Thailand.
The other interesting place to visit in Nakhon Pathom is the royal palace of Sanam Chan, a curiosity which dates from the beginning of the last century. What with the statue of the dog elevated to hero status (simply as a mark of affection) and the main building which looks like a Disneyland castle, there's plenty to look at for a while. The garden is superb, the museum is interesting, if a bit too patriotic, and a day spent at Nakhon Pathom goes quickly.
At 120 meters tall the stupa of Nakhon Pathom is the tallest in the world. It contains some relics of Buddha. All that was needed to make this site extremely sacred to Buddhists.
Having left Bangkok, I went there during my journey around Thailand, on my way to Kanchanaburi. A 2 hour stop is enough for the visit. Remember that it is essential to walk clockwise around the stupa. I felt very small next to this giant! While going round I found a small museum, a little temple, some caves with statues of Buddha, and a chapel at each point of the compass. They are called viham.
In short, make a short stop at Nakhon Pathom, a not-to-be-missed site in Thailand.