Air-con 2nd class buses depart from stall 99 of Bangkok's Northern Bus Terminal (Moh Chit) every 30 minutes or so and cost 43 baht (Nov 2007), no advance ticket needed. You may also be able to hop on board along the way outside MRT Phayon Yothin station. Being a second-class bus, it stops at every bus stop, shopping mall and grilled chicken stand along the way, so the journey takes the better part of two hours.
Bang Pa-In is on the northern line to Ayutthaya and Chiang Mai. Three trains per day in each direction stop at Bang Pa-In, and the journey from Bangkok takes 1.5 hours. You'll have to make an early start to catch the 07:00 from Hualamphong though.
There are no scheduled services, but many of the tourist cruises to Ayutthaya stop at Bang Pa-In on the way.
A taxi from Bang Pa-In to Ayutthaya costs a fixed 150 baht.
Tuk-tuks of various interesting shapes and sizes shuttle between the BKS station, the train station and Bang Pa-In Palace for 30 baht (Nov 2007) a pop.
Bang Pa-In's number one sight is the Bang Pa-In Palace , also known as the Summer Palace. Originally built by Ayutthayan King Prasat Thong in 1632 but abandoned after the sack of Ayutthaya in 1767, the site was partially restored by King Mongkut (Rama IV) in the 1850s. The site as it stands today, however, is largely the work of King Chulalongkorn (Rama V), who expanded the area into a Versaillesque garden filled with European-style buildings in 1872-1889. Last restored in 2001, the palace and its grounds are maintained in immaculate shape and well worth a visit. Entry 100 baht, open daily 08:00-16:00. The grounds are not too large to be covered on foot, but you can also rent a golf cart to scoot around in for 400 Baht for the first hour and 100 Baht per hour after that. As at all royal sites, proper dress is required, but you can buy a 100 baht wrapwround skirt from the stall in front of the entrance if needed.
Across the river from the Palace is Wat Niwet Thamprawat, another of King Chulalongkorn's European follies, built in 1878. This is an active Buddhist temple cleverly disguised as a Gothic church, down to the spiky eaves and stained glass windows. Getting there is half the fun, as a basic motorized cable car swings visitors across the river! The cable car station is hidden behind the Bang Pa-In parking lot, which explains why the temple doesn't get many Western visitors. Free entry but donations welcome.
Exit from Bang Pa-In palace is through a giftshop selling touristy junk like flourescent baseball caps emblazoned with "Phuket" in large letters.
The Bang Pa-In palace grounds have a number of simple cafes selling soft drinks and snacks (10-20 baht). The best views are from the one inside the Tevaraj-Kanlai Gate, right opposite the Thai pavilion.
There are basic but rather unappetizing food stalls just outside the Bang Pa-In entrance. Central Bang Pa-In near the BKS station has more of the same, as well as the obligatory 7-11 across the road.