The rooftop garden sounds lovely. Lots of things will grow in pots or grow-bags. You don't say which country you are living in, but I would suggest tomatoes and courgettes which will grow in pots, as will, scallions, come and cut again lettuce, everlasting spinach, and lots of greens like mizuna and land cress (full of vitamins and calcium). I would also go for goji berries, they will grow in tubs and also cape gooseberries. You can also grow soft fruit such as blackcurrant in a tub, or train a bramble such as tayberry or blackberry to go up a fence or trelis. If you want to grow herbs, then you need to add a 50% mixture of grit or horticultural sand in the pot, as they grow best in this ratio.
If you don't mind telling me which Country you live in, I will have a look at my Horticultural books for you and see what grows best in your 'zone'.
Blessings
Raw Gardener.