Potentilla is a maintenance-free shrub that likes sun - it flowers for ages (comes in various colours - white is nice) and stays at a 4ft-ish dome. You could put that on the back corner. Maybe a small tree, such as a malus (crab apple) or robinia would be nice, somewhere in the middle, just to give it a bit of vertical. Or for that matter a columnar cypress, elwoodii, or small thuja if you want it evergreen. You might consider a prostrate juniper to front the border next to the grass - they're pretty quick to grow and cover ground maintenance free. Or edging with a lavendar 'hedge' - this would just need a quick clip after flowering to keep it compact. 3-plant clumps of hardy geraniums would work well - just chop them back to ground at the end of autumn. I personally like the blues best. These too flower for ages and don't need messing about with, so lots of bang for your buck. To separate your roundy clumps you could use crocosmia, daylilies (hemerocallis), grasses, or even irises. You could also stuff in a few clumps of 5 or 6 allium 'purple sensation' bulbs for accents. Anywhere u like. Again, easy to maintain - just chop off every when they go brown. For mid-height, sedum spectabilis (iceplant) is a good easy do-er which likes sun, drought and general neglect. Flowers late in the year too, so good for continuing the show. And I'd spray that fence dark brown - shows off the greens of plants better. I'd keep foliage to greens - its too small a border imho to start mixing in varigation and purples, though you'd get away with something like euonymous fortunei 'emerald gaiety' (small evergreen shrub that'll almost climb a fence if put next to it), particularly if you had some white flowers elsewhere, as the variegation is very white, or 'emerald and gold' if you had yellow flowers, as the variagation is very yellow. If you wanted another shrubby-job, euphorbia 'wulfenii' would be lovely, maybe at the front behind the wall.
I'd stick with getting your contrast between neighbouring plants through their leaf shape and size rather than mixing too many different colours. And I'd plant the non-shrubby stuff in triangles of 3s or 5s - you need decent clumps if it isn't to look 'bitty'. All of the above are quick to establish, easy to maintain (and to find in garden centres), and won't go mad and take over the world! Hope that helps xx
