I know you said you've decided against a bunk bed, but check out this bed from ikea. It's called a reversible bed. You can construct it as a loft bed, with play space or storage underneath, you can make it a typical bunk bed with matresses on top and bottom, or you can make it a bottom bunk only, with the "starry sky" tent on top for more of a canopy/tent feel. And one other cool thing about it, as if the versitility of it isn't enough, is that it's not as high as a typical bunk bed, so for little ones, it's safer, plus, for older ones, they're not as close to the ceiling!
http://www.ikea.com/us/en/catalog/products/10123996 (BED)
http://www.ikea.com/us/en/catalog/products/20116207 (TENT)
My son has this bed (he's 4), and it is so cool. He LOVES it. He plays endlessly in the bottom of the bed, because with the matress on top, he plays like the underneath is his cave, house, tent, etc, etc. The only thing we did to make it a little safer, since my son was below the recommended age for the bed (I think it was 6), we screwed handles onto the sides of the ladder at the top, so he would have something more substantial to hold onto while climbing up and down, instead of having to hold onto the bed itself.
Have fun choosing!