I've mentioned before that my wife and I moved into a new house back in late May. Our kids aren't old enough that we can trust them coming up and down the steps just yet. I wanted to have a way to secure a baby gate to the top of the steps without permantely damaging the brand new stair banisters. My wife found this on Pinterest. It looked pretty simple, so I thought I'd give it a shot.
2x4 (will be cut in half, so measure accordinaly)
Stain of your choice (bonus points for using the same one as your steps and floor)
Step 1: Measure how high your stair banisters go up and cut the 2x4 to around that length on each side.
Step 2: Stain the wood (optional, but looks better).
Step 3: Place felt pads near the top and bottom of the wood. I also placed them on the bottom to protect my floor. The goal here is for the wood not to touch anything.
Step 4: Steady the wood against the banisters and wrap 3 or 4 zip ties around it and pull it tight (repeat on both banisters). You then should cut off the excess zip tie.
Step 5: You'll want to get a baby gate that is actually drilled into something rather than just using tension rods. This is the one we used. Install the gate using the newly attached wood posts as your mounts.
Outside of the actual gate, I probably spent $25. This is a great solution as it keeps your kids safe without putting holes in the wood. Some people might only have a stair banister on one side and have drywall on the either. I have no problem putting a hole in drywall as it's pretty simple to patch/paint later on.