A number of interesting places are within an easy day trip of Bangkok. Perhaps the most interesting is a trip to the old capital of Ayuthaya, which can include stops an the Royal Arts & Crafts Center at Bang Sai and the Bang Pa In summer palace. In addition to Ayuthaya, here are some other ideas:

Ko Kred

Only a a short distance up the Chaophraya River from downtown Bangkok is the island of Ko Kred. The island was created almost 300 years ago when the ruler of Ayuthaya had a "shortcut" dug to shorten the trip up and down river. The island's claim to fame is its delicately patterned earthenware pottery, but the real attraction is the chance to see a glimpse of village life. You can easily reach the island on your own by boat.

Wat Chalerm Phrakiet and Chalerm Kanchanapisek Park

On the banks of the Chaophraya River north of Bangkok, King Rama III built a large and impressive temple to commemorate his mother, who once lived in the area. The temple was built within an old fortification built by King Narai of Ayuthaya in the seventeenth century. The outer wall, in particular the side facing the river, still resembles a fortress.
 
Next to the temple is a relatively new park built to commemorate the 50^th anniversary of the previous king's ascension to the throne.

The Ancient City

Just outside Bangkok, on the old route to Pattaya, a Thai tycoon conceived a sort of "Cultural Theme Park" where Thais could learn about their country's architectural history. The huge park easily takes a full day to visit and enjoy.

The Buddha's Footprint

One of the most sacred sites in Thailand, located in Saraburi province north of Bangkok. Seemingly out in the middle of nowhere is an ornate temple covering a natural formation believed to be a footprint left by an incarnation of Buddha.

Lopburi

The modern town of Lopburi is about two hours' drive north from Bangkok. Occupied since at least the sixth century, Lopburi has been an important city since the Khmer empire in the tenth century. Later it was used as a second capitol of the Ayuthaya kingdom by King Narai. The remains of his palace is one of the main sights. Also of interest is the Chao Phraya Wichayen, used as a residence by Narai's Greek-born chief minister. Also of interest, sort of, is the Kala shrine, a temple located in a traffic circle near the railroad tracks. The temple itself isn't very interesting, but its over-run with monkeys who have grown fat and even rather agressive on tourist handouts. Don't say we didn't warn you!

Phra Pathom Chedi

Nakorn Pathom town is about one hours' drive due west of Bangkok. The largest Chedi in the world, this monastery is the site of a huge temple fair in late October. Its also extremely old, having been originally built in the sixth century, although added to and "improved" many times since then, which is how it reached it current girth.