tomatoes

At the end of the season, any green tomatoes can be put in an enclosed space (a drawer for example) with either a ripe one or a banana. The principle is these give off a chemical which triggers the unripe ones to ripen. If you've got a spare banana, you could try putting it under a tomato plant and see if it has any effect

In the past I have used a teaspoon of epsom salts, this ripens the tomatoes.