They say you need to have photos, and all the other passengers had them. However, both times I went, I forgot to bring them. They just frowned at me and then gave me the visa anyway. Do try to be first off the plane if you can, and go straight to the visa-on-arrival counter to the right when you enter the building. The wait can be either 2 minutes or 30, depending on how many other passengers from your flight are in front of you.

As for climbing, it's definitely not necessary to climb anything (other than a few low sets of steps) in order to enjoy the beauty of the temples. You miss out on a few views but you'll still see most of the good stuff.