Build Your Own Baby Gate. This is great for the bottom of the stairs, or the top, if you used a more child-proof latch design. Maybe just putting the latch on the side the kids aren't supposed to be on would keep little fingers from opening this. I REALLY love how it looks--and it's really just a half door. Perfect.
Try removing the door to your toddler's room and replacing it with a swinging gate. It's better than keep the door open all the time and far more interesting than keeping it closed... and the best part it can keep them in the room when you don't want them roaming the house.