If you're using the greenhouse for starts/seedlings that will be moved, build up with multiple layers of shelves.
If you are going to build planting table beds, Fewer wider beds are better than multiple narrow beds. One 30" wide walk way is plenty. Two 15 are not... Plus the width of your lumber.. not efficient use of space.
What do you have set up to vent it, because overheating is one thing you can't come back from. Everything else is up to personal preference.
You can have one sunny 75 degree day in April and fry everything if you don't have a plan for venting. I thought I'll just keep an eye on the weather.. had one unreliable forecast.. a 62 degree expected high turned out to be 70+ and by the time I got home months of work was cooked and an entire growing season wasted.
