All of your answers are Yes, yes, yes.
Yes- depends on the sun.
Yes, you can put a box on top of the cement.
Yes, you can grow grass, flowers, veggies, or whatever.

The way the economy is this yr. I think I'd be tempted to grow something I could eat. Maybe even one tomato plant & enuff lettuce for a salad, ha.

Good Luck,
Gumby_CT

eww peppers(yummy)!! and some.....yep that's what i'd do, why not plant vege;s AND flowers in the same box? i don't see why ya can't? in fact on some sites they have companion plants, they are plants that keep bugs away from your veges ya want. you could grow herbs also, but if you grow mint, do it in a container, make sure to check if the ones you grow are aggressive. :'>> but heck why not?? if you know how to can or want to learn then you've got the perfect thing for it :'>> if ya got the time, LOL mom said things are gonna get really tight, she's a trucker and they are getting tight on the loads, so she warned us ahead of time, thank goodness for mom!! LOL i have to learn how to can!! and the freezing and all that this year. :'>>

If you plant grasses, you will have to mow it, that alone would make me pick something else. Many plants have lovely forms and are fine for "visable" gardens (besides anything's gotta look better than all concrete). Cucumber vines can be used like a morning glory (grown on porch posts, along a fence etc.), many "bush" squash forms a central plant (think of elephant's ears) peppers are both beautiful and handy, strawberries form a nice "groundcover" and give fruit as a bonus. Herbs are both useful and subtle. You can always toss in some pots of flowers to "pretty" it up. Best bet would be go to the veggies in the garden center, look for nice shapes, and then decide what your neighbors won't mind seeing.

Thanks for the help -- at least I know where to start!

There are ornamental grasses which don't have to be mowed, I have several that are beautiful. I usually cut them back in late winter/early spring. Evening sun on concrete would be very hot. you will probably need to plant things that love/need sun like tomatoes, peppers, okra, sunflowers, herbs. You will need to be able to water your plants, obviously, do you have a hose nearby? I've never done square foot gardening, but I do grow lots of stuff. you would have to find out how deep to fill the boxes. Good luck. Happy gardening!