campanula is nice. Its low growing with purple flowers, it will spill over the edges of the tyres and it stays green in winter. It doesn't get out of control like some ground covers do.
as someone else said, violas and pansies will self seed and pop up all over the place giving bright splashes of colour.
coreopsis is lovely. Its tall and bright yellow. The double varieties are especially nice and it likes hot and dry conditions.
DON'T plant forget me nots unless you want to spend the rest of your life weeding them out
Spring bulbs would be nice but its too late to plant them now - maybe leave some space for planting them in autumn.