We spent a couple of days in Bangkok which was amazing, but wanted to head to a more rural area for a couple of days. North west there's a couple of national parks which we decided to visit, Erawan being one of them.
We actually did a 3 day trip to the area which cost us 3000 baht each (about $96 or £68) but you can look around for shared minibuses for about 1500- 2000 baht return for the day if you wanted to do it yourself.
The waterfall itself has 5 levels, each of which you can swim in which is a good day out, and not too busy either.