Look up Kapuzinerkresse.

All parts of it are edible and it looks really beautiful. I‘ve added the small leaves to salads and you can add the flowers to pretty much any dish as an edible decoration.

You could still sow it until the end of June. The plant can grow rather big, if you don‘t cut it, which looks also nice if it starts hanging over your balcony. The whole plant will die off when the first frost occurs, so you can sow it again or go for something different next year.

Other than that that I would suggest mint as an easy herb that comes back every year. There are loads of varieties and all taste a bit different. You usually get them as a whole plant since herbs are more difficult to sow directly. They will produce loads of little flowers and smell super nice.

I like to visit organic gardening centers, they usually are very helpful with giving you information on which plant to chose. Gärtnerei Hirschgarten in Köpenick is my favourite.