Mr. built this for Jr.'s crib mattress. It is quite cozy, and significantly cheaper than an ikea toddler bed.
I have plans to paint it, but it looks nice natural, too!
EDIT (Mr.): Full credit should be given to K. Christopherson for the plans (which I modified as necessary - Jr.'s not quite ready to be up on legs, he still slithers out of bed now and then). Plans are here.