This is a great idea... The thing is that you should think about the weight that you put because is can cause damage to the builting. Other than that the fountain is perfect and you can put some outdoor sofas with colorful pillows. Also you can make a small bbq.

If you could post pictures that would help, also make sure of your building's codes/rules, keep in mind weight, drainage, exposure (sun/wind/light), privacy concerns, lighting, etc. and consider getting professional design help for someone who specializes in roof gardens. You don't have to do everything all at once but a design professional could help you clarify your vision and give you ideas you may not have even thought to incorporate. Best of luck, 800 sf of outdoor space is wonderful. Don't forget you've got to get everything up to this space too.