My husband made this fort for the girls by planting willow stalks at four points and then bending them to meet and tying them together. There are a ton of YouTube videos to help. It will, theoretically, fill in (the stalks have just started to sprout) and there are plans for windows and a tunnel, but until then it's fun to tie up some play silks for walls. We hung some crystals and shells inside and I kind of want to live there. Cost: $0
Or, another option is this little GardenFort, which comes with three kinds of seeds and is $70 on magiccabin.com.