Mexican oregano comes to mind. It doesn't set seed for lack of a proper polinator and the clump grows larger but doesn't spread. It might flop in partial shade so don't plant it too close to the path, but part shade doesn't limit its constantly being in flower. It blooms in all seasons except winter and even then it retains its leaves which I use in cooking. It's a wonderful drought tolerant plant. Check it out.

Here is a link that might be useful: Google images of Mexican oregano.