About five months ago we got a kitten.
Within a month or so of living with us, he became pretty desperate to go outside. Whenever someone opened the back door to go to the yard, he made a mad dash for the door. Now that he's old enough to go out and has had all his immunizations, we built him a little ramp out of our upstairs window so he can come and go as he pleases.
It's a little plank of cedar panelling onto which we stapled some balsa wood grips. It's attached to the bougainvillea at the window and wedged into the thuja tree. He walks out to the tree and then scrambles down to the yard.
He heads out for some playtime during the day between naps and always stays inside at night and to our glee has given up his litter box entirely.