certain low growing campanula will grow quite well in cracking concrete

If the cracks are big enough thymes and sedums may grow OK

Best to start by sowing seeds directly into the cracks so that the roots can get a good purchase. Erinous alpinus (Fairy Foxglove) could be suitable.

You won't go far wrong with poppies.

The yellow Dicranostigma franchetianum are particularly nice and remain low growing in comparison to the traditional reds seen most often.

Try Erigeron karvinskianus 'Profusion' and how about Alchemilla mollis in the shady areas? Or try poking cuttings of Helianthemum (Rock Rose) in. Aquelegia and Nigella would grow there too, see if someone you know has seed on their plants now.