In addition to the UTH and thermo above, you will want:
1) Bedding - I like aspen, but you can use other materials. Avoid pine and cedar, as they have oils that can become toxic
2) Hide - should be cozy but large enough to accommodate growth. might want multiples for both hot side and cool side, but the hot side one is most important. Can go cheap or fancy here as you wish.
3) Water bowl - snakes do drink, and having a source of water for soaking and humidity control is important. Clean and change regularly (whenever it is dirty, or every 2-3 days). Make sure it is hard to tip over and big enough for snake to occupy if it chooses. Use filtered or distilled water, or use dechlorinator on tap water - chlorine is not good for reptiles.
4) Humidity gauge - not required, but useful, especially in winter when many homes get pretty dry.
5) Decor - this is up to you, but corns like places to hide and crawl through. Empty toilet paper or paper towel tubes actually work nicely and are economical if not aesthetically pleasing, but you can put in whatever you like and your budget allows.