The only indoor snow place I know of around Manchester is Xscape but that's outside Leeds (the distance isn't too far if you have a car though - straight down the M62). One cool place is Knaresborough where there are remains of a castle and Mother Shipton's Dropping Well (which is worth a visit). It's a bit up the M1 past Leeds.
If you head east a little way you come into my next of the woods: Lincoln (the City of the Imp ). We have a castle and a cathedral across the square from each other. The castle also became a Victorian Debtors Prison which you can walk around. In December there is the Christmas Market (over four days and nights (or so) which is a sight to see. Further east is Tattershall Castle which is a good visit in itself but if you're lucky you'll also see the RAF Battle of Britain Memorial Flight in the air over the castle as they operate from the nearby RAF camp (RAF Coningsby):
Nottingham is also worth a visit, although the castle that is there isn't the same one as in Robin Hood's days You can visit Ye Olde Trip to Jerusalem though which claims to be the oldest pub in England and, of course, to the south of the city is Warhammer World and the HQ of Games Workshop.
At least two conventions are happening: Dragonmeet in London in December (which I won't be at) and Conception near Bournemouth in January/February (which I will be at - for the weekend only). In fact, there's on in Sheffield the week after you arrive (Furnace) which has won the UK Role Players Best Convention 2011 and the UK Role Players Golden Crown Award for Best Convention over the last 10 years! It's held in an old garrison building (which is now a hotel) and you can play games in the cells!
Think that's about it for the moment (I should be working!) Drop me an email if you like and I can see about racking my brains for more information.