If you are in London during the Christmas period, Winter Wonderland in Hyde Park is definitely worth a visit. It is fairly empty from Monday to Thursday and then gets quite busy but although the queues look impressive they don't actually last very long.
The Attractions - The park has a lot of great rides for the younger kids just like it has some very decent attractions for the more mature crowd. Santa Land is full of games that will cost you a fortune and you most probably won't win anything, but let's face it you are not here to win but rather to have a fun time. I highly recommend the Racing reindeer - there I said it.
The Food & Drinks - Just like any typical - or not so typical - Christmas market you can get your Mulled wine - with or without a series of other alcohol shots in it. You also have your hot chocolate (I would definitely go with the Baileys Hot chocolate) and German beer.
As for the food, there is just about anything you can think of from savoury to sweet; hot dogs, pulled pork, skewers, pancakes, waffles, potato crisps, corn on the cob, pizzas, candy floss, pop corn... Basically if it is bad for you yet oh so good they will have it.
The Atmosphere - I went on a Sunday evening and had a blast but boy oh boy is the place crawling with drunk teenagers and rowdy men which is a bit of a shame but we just saw it as an additional attraction.