For sun or shade? If shade then begonia would thrive
Sun a some crazy daisy, pinks, million bells, chocolate cosmos or Bergamot all are easy to grow and have the natural wild look.
I would start by replacing the soil as the old plants will have drained all the nutrients from it. Replace with a mix of 3 parts compost to one part peralite and add in a hand full of slow release plant food pellets.
The garden centres are overflowing with plants this weekend, go and have a browse. Be prepared to water regularly even if it's raining. If you like salad I'd do at least one with salady stuff and maybe a little tumbler tomato. While you are there get some sticky copper tape, use this round the edges of your planters to stop slugs and snails climbing up and eating your veg.