The cheapest thing in the long run would be a covered walkway with travellators. For the most part it would just require sacrificing a couple of spaces in each row. Big initial cost but tiny ongoing costs compared to the monorail or operating 3-4 trams continuously and in the summer months you could get a few stalls out selling refreshments. 

The biggest problem with travellators would be that there would be a far bigger rush of people leaving the car park at once rather than it being spread out over an hour or so.

Can't really see a citywalk style plaza of expensive themed restaurants working given the seasonal nature of the park and the majority of visitors having hours of driving ahead of them when leaving (that said, a coffee shop and fast food outlet would probably get full at closing time and they could charge service station level prices).