There are many ways to add doors to a greenhouse. My main T-shaped greenhouse has three wood-framed end walls; one is framed around a door, and the other two are framed around windows and high-powered industrial fans. Because the plans for this improvised greenhouse still had loose ends and unanswered questions (e.g., “How long will a kid's terrarium hobby actually last?”), I created a zippered entrance using ZipWall Adhesive Heavy Duty Zipper Kits ($24.98 for two at Lowe's) to create an easy access point. These nifty little devices are used for painting rooms, isolating areas for mold removal, and more.
Ventilation is essential to expel hot air, but depending on your climate, ventilation alone may not be enough to fully control the greenhouse temperature during hot weather. Use exhaust fans on end walls or removable shade or landscaping fabric for parts of the structure. Commercial greenhouses often come with roll-up sides and are available from suppliers such as Grower's Solution ($67 for an 8-foot side). Or you can DIY a basic version using PVC pipe. I built one of the greenhouse walls out of landscaping fabric to allow for a little ventilation. Plan your furniture placement so that a south-facing window serves most of the plants and keeps the work area cool. Repurposed gazebo greenhouses are by their nature fairly small, but if you have the space for a comfortable lawn chair, they can also serve as a great place to take a nap on mild spring days.