P.K.
If it's portable you're thinking of, a sleeping bag works great. When my daughter sleeps over at Gramma's, she brings her sleeping bag and sleeps on the floor on a puffy blanket underneith. She loves it and really, when kids are little, they can pretty much sleep anywhere. (they're not like us, waking up with pain if we sleep on the floor.) Having a sleeping bag is sort of a novilty for little kids anyway. Plus, you can get one that's a cool theme, spiderman or whatever, and you can even have them pick it out.
As far as a perminent solution, We put my daughters mattress on the floor (Less of a drop if they were to roll out), and eventually,...we added the box spring underneith. We will eventually add a frame to it. (We skipped the toddler bed entirely and opted for a regular twin mattress. We thought it was a waste of money for a toddler bed only to upgrade to a regular bed in a year anyhow.)