nasturtiums (you can eat the flowers in salads)and calendula (pot marigolds) are colourful, grow easily from seed and are pretty hardy.
Both will set seeds which can be gathered and used for next year.
You can buy a lavender plant for a few quid and it will last ages, if you want hardy herbs go for rosemary, mint, lemon balm, sage and thyme, all will survive all but the most savage of winters and you shouldn’t pay more than £1-2 for a plant that can just be planted straight into the ground.
For a few quid more get a bay, hardy evergreen herb which grows well and is frost hardy.
If you want easy veg, go for courgettes (if you have the room), radishes, tomatoes (cherry variety may be easier and are more blight resistant) and a packet of salad mix.