Well, everyone's beaten me to it!
I'd pretty much agree with it all - Jorvik is, I think, over-priced, and for someone with a strong interest in the subject, information-lite. But there's always a queue outside... I think once you've done it, you don't need to do it again, and you won't miss a huge amount if you skip it.
For iconic picturebook views of York, walk up the Shambles (medieval jettied houses that almost meet overhead. While you're there, marvel that in the 60's when they built the hideous concrete building on Stonebow, they had a plan to demolish the Shambles and build on that too), and then up Petergate towards the Minster - the Minster looms behind the shops wonderfully. If you can arrange snow for that, it's even more picturesque. The Walls also afford some fantastic views of the Minster and city. Clifford's Tower also gives good views from the top (it costs to go in, can't remember how much, it's English Heritage). The tower of the Minster can be climbed, if you don't mind 200 odd spiral steps... I think for a day trip, just going in to ogle at the Nave will be enough. Look up at the bosses in the South Transcept designed by Blue Peter viewers after the big fire.
Talking of which, if this sort of thing amuses you... Down the Nave, between the tower and the West end, there are bosses in the middle of the ceiling where the spans meet. They depict biblical scenes. About half way down, if my memory serves me correctly, there is one depicting the accension of Christ, depicted by a carver in the way he reckoned he'd see it - just the soles of Christ's feet as he rises up above!
I too could spend a day in the Railway Museum, and indeed have, many times - but the great thing is that it's free, so even if you only nip in to look at Mallard, you've had your money's worth. How are you arriving? If by train, the Railway Museum backs onto the station. If by car, use a Park and Ride to get into town - much better than driving in and trying to park.
There are tonnes of cafes, restaurants and coffeeshops. If you want to do Betty's but the queue is long and outdoors, nip up Stonegate to Little Bettys - a more Olde Teashoppe atmosphere, over a tea and cake shop, and while there's often still a queue, it's indoors!
