Also, in the UK at least, look for chains specialising in lunchtime food (particularly sandwiches, sushi, etc) selling off their food half price from 3-4pm onwards.
You can also find some very decent best-before-end-the-same-day food and pre-prepared meals (aka "salmonella corner") significantly marked down - some great bargains to be had at the higher-end supermarkets, particularly on Sundays when they need to close at about 4-5pm.
If it's a nice day - get a disposable barbecue! You can find one for £3 or less, and pick a spot to use it (just make sure it's allowed, plenty parks ban them as they damage the grass...)