Campanula poscharskyana. Low growing, spreads by underground runners and also seeds around. Pretty blue flowers all summer. If you want something taller in amongst try some groups of wild foxgloves - these are usually biennials by but they usually seed around. Once planted these should more or less look after themselves as long as the inevitable weeds are removed before they join in the seeding around party. Good luck.
(This is assuming you are after something you don't want to spend a lot of time looking after?)

welcome to GOY too.
Yes the suggestions from Steragram are good. also consider annuals like California poppy and Nigella. They will self seed.
also think about pot marigolds [calendula] some of the low growing herbs like thyme. Then there are sedums that are small and creeping that would come back year on year.

Delosperma cooperi and kin, if the drainage is good.

Campanula carpatica (or similar variety). They are low growing rockery plants which spread and will cover the ground with pretty blue flowers in summer. Also try the sempervivums as ground cover too.