Bean bag chair replacements are doable. The problem with bunk beds is that the kid needs to fit on the bed. They will lie on a particular part of the bed image, and with a bunkbed that might create some very odd effects where the kids body would appear to be lying over the supports for the upper bed. Also, no kid would lie on the top bunk ever, because that's not an area programmed for sleeping postures.

When you say "something besides bean bags" do you have anything specific in mind?