Even if you don’t have a garden, anyone can grow a few herbs and lettuce leaves in a pot on a windowsill. With Lettuce you can keep it growing and growing by just cutting off what you need to use. It’s a very cheap way of getting salad and kids really enjoy growing their own. Try cress planted in a washed out empty egg shell (the kids can decorate the shell so there is free entertainment thrown in too).
If you do have a garden or balcony try tumbling tomatoes in hanging baskets. I found most of my hanging baskets in skips, when they go rusty people just chuck them out, but with a wirebrush and old growbag as a liner you can grow lots of different things.
Look out for seeds at the end of summer for next year. Places such as Wilko’s often have them reduced to 5p a pack. You can also save your own seeds for next year by drying them out (sometime I’ve found this works and some times not!).