porchulaca and purslane are my two favorite fillers...
Yeah, I've got some at the head of my planter, the low part that's nearest the street. Just gives some color and green, and you don't have to think about it.
It was 104 degree when I got home yesterday, so I'll wait until Saturday, early morning to put them in. Got a lot of dead heading to do too.
neko, phlox is a perennial around here, I've got some as the border of one side of the main part of the garden, been there for at least 3 years.