campanula grows everywhere and anywhere and you can get a few different colours.
herb robert gets little pink flowers and grows quite quickly.
nasturtiums are colourful grow quickly
hollyhock takes a while to grow I think but it comes in a lot of colours, grows tall and is quite pretty.
lavender is good, and you could get a flowering thyme.
you could also try non-flowering herbs with different coloured leaves for a bit of variety. Variegated lemonbalm is pretty, and you can get cream oreganos and lemon thymes (lemon thyme is the most delicious smelling thing I've ever had near a nostril).
ETA: just thought I'd mention, I think that's a bluebell in your photo and I'm pretty sure those aren't bunny safe. I just spent ages digging them out of my garden in case the buns decided to have a nibble. Such a shame as they're so pretty.