A greenhouse is important to get your garden activities start earlier. I love the idea of fold-down or convertible green house. In night or cold days, your want the plants to be covered, but when it is warmer days, you can open the cover. I found this one on sale on montanagifts linked here.
With some additional research here are a couple of DIY versions.
Instructions for this convertible greenhouse linked here.
And tutorial for this fold-down greenhouse is lined here.
If you have build a different version, please share it in the comment box. I am sure it would be inspirational to our fellow readers.