Try seed varieties of vegetables you can't buy in shops, or try the space-saving or unusual ones. Here are a few possibilities, taken from seed catalogues.
You will find McGregor Seeds' Cylindra carrot-shaped beetroot in garden centres as seedlings ready to plant. Their Parisian Market baby carrots are ideal for school lunches and will be ready to eat in 75 days - with 1000 seeds in the packet, there's plenty for children to experiment with - and the Easy Peasy Pea seeds also sound like fun.
From Kings Seed Catalogue there's Gold Rush, a bright yellow zucchini, or how about a packet of cocktail mix tomatoes which are easily grown in pots. Try the Albino beetroot or Green Sausage tomatoes just because they're strange. Kings also has a huge range of basils, from mini to giant, green to purple.
From Yates Seeds, there's the Greenway lettuce, specially bred for warm summer conditions, or try baby beetroot or purple beans.