The Erawan Falls are contenders for the most beautiful waterfalls in Thailand, and a must-see if time and budget allow. Entrance fee is 300 baht for foreigners. The falls are composed of seven tiers, all of which are picturesque and great for swimming. Plan to spend at least two hours hiking plus the time you want to spend swimming in the falls.
Don't come unprepared. Wear a swimsuit and bring sunblock, since you'll want to have a dip in the turquoise pools on most levels. Don't forget to bring a towel. When swimming, watch out for fish feasting on the soles of your feet. They won't hurt you and are only looking for a meal on dead skin cells, but the feeling can be disconcerting.
Everyone can do the hike, but don't underestimate it. Good shoes will make the trip more pleasant, though flip flops are commonly worn. At the highest levels, one may have to walk through shallow water. The first four tiers are relatively close together and the walk is very straightforward. For the more adventurous, there is a large rock at the fourth tier that can be used as a water slide. Beyond the fifth tier, the hike will become slightly more difficult. The sixth and seventh tiers are not far from each other, but the paths are not well defined at this point, so be sure to look for the hard-to-spot signs. Additionally, beware of hornets at the top tier.
Bicycles can be rented at the entrance for 20 baht/hr, however you can only bike to the first level, which is only a 5 min walk, so they aren't really useful. Many Thais don't go further than the second level as beyond this food and beverages, except a water bottle after leaving a deposit, are not allowed.
If you walk on the right hill side of the road leading to the park gate, rather than the road itself, you will pass nice bamboo forest and you won't be asked to pay entrance fee, since they collect it only at the toll gate if you enter by main road.