Local buses run approx. every 20 minutes (06:00-19:00) from the nearby provincial capital of Nakhon Ratchasima (also known as Korat). The local bus may stop on the road leading to the Khmer ruins; if you see a Khmer temple, get off.
From points north, disembark at Talad Kae, and wait for the local bus as above. The fare from Nong Khai is 200 baht to Talad Kae, and then 13 baht from there to Phimai.
The town itself is quite small and can be easily explored on foot. Visiting the Banyan Tree requires renting a bicycle, motorcycle, or taxi.
Phimai is famous for the Phimai Historical Park and its Khmer temples, among the best-preserved in Thailand and similar (if much smaller) than those at Angkor Wat. It opens daily from 07:30 and costs 100 baht for foreigners. Parts of the park are older than Angkor Wat in Cambodia. Includes Khmer buildings, sculptures leading to the shrine, restaurant, and gift shop.
The largest banyan tree in Thailand is located approximately 2 km outside the centre of town.
The night market takes place every evening in the center of town (16:00-21:00). The Monday market takes place each Monday on the outskirts of town, near the new bus terminal and the Phimai Inn. The 8 market takes place on the 8th, 18th, and 28th of each month beyond the South Gate of Phimai, alongside the road leading to Baitey's and the Vocational School.
The night market offers cheap food for more adventurous visitors (don't expect to be able to identify everything on display). Baitey's Restaurant, in its new location 3 km north of the city center, serves popular Thai and Chinese dishes, along with a few Western staples, in a scenic open-air setting. A small restaurant in front of Phimai Historical Park ; Pornsilp is another good choice with an extensive English and Thai menu.
The most obvious choice for late-night entertainment is a restaurant next to the 7-11 and across from the night market, in the center of town. Serving beer and whiskey until 01:00-02:00, it's a popular spot for young Thais (and foreigners) to gather for celebrations and musical performances. A young man plays guitar and sings every evening and will even play a handful of popular English songs if a foreigner is present. There are also various karaoke bars scattered in and around the city.