The £5 fee doesn't apply if you check in online with hand baggage only. For obvious reasons, it has technically to be possible to get a flight for the advertised price.
For instance (looking at Ryanair's home page today) they're advertising Stansted to Billund (Denmark) for £3 (offer ends midnight tomorrow).
To get that fare you need to choose a day when there are £3 seats available (at the moment. for example, you could fly on any day in June bar the first sunday), take hand luggage only, check in online, and pay with an electron card.
So, if you felt so inclined, you could take a morning flight (07.55 arr 10.25); visit Legoland for the day (it's right beside the airport); and return (18.45 arr 19.15) for a grand total of £6 per head.