Take your own snacks, cereal bars are really good to keep you going through the day.
Eat as much as you can at breakfast, I find that after a good breakfast we're not very hungry at lunchtime, so cereals/fruit/crisps see us through to late afternoon/evening.
Don't buy snacks in the park, they're really expensive, but I think that a cup of Hot Chocolate warms you up and fills you up so they're worth the money, or you could take your own. I always take a kettle and the individual sachets of hot chocolate and coffees with us for the room, but if you had a small flask I don't see a problem taking it into the park, although taking food/drink into the park is against the rules, I've never heard of anyone having theirs taken away.
Eat your main meal in the Village in the evening, Annette's and Billy Bob's are both very reasonable and if you're really struggling there's McDonalds.
If you're intending to buy filled rolls or baguettes, go to the cafeteria inside the train station, the ones there are lovely and fresh and again cheaper than inside the park and the little newsagents/tobacconists in the station sells crisps, biscuits and chocolate bars.
Just had another thought and I don't condone people pinching from the breakfast table, but if you took one or two rolls from breakfast
and had taken some instant soups or instant pasta pots with you, you could pop back to the hotel for a nice warm filling lunch, providing you have a kettle.
Accept that you can't go buy lots of souvenirs and just get one special one, a Christmas bauble would be nice, something that you'll bring out year after year.
We had a fabulous Halloween trip in 2004 on a budget, despite not eating in fancy restaurants it's still one of my favourite trips.
