We "budget" for extras when we travel (by "budget" I mean if there is something expected or unexpected that we want to do and it costs more, we just do it and don't care what it costs, without going overly crazy about it). We rationalize it by saying we can't go back in time and add that foofoo drink at sunset on our ship balcony, or that tour of the Paris catacombs that scared the crap out of our then 8- and 10-year-old kids, in ten years when we think we can afford it better. So far so good, no regrets.
Next year we have three big trips planned, two to Europe and one to Hawaii. I might have to modify this philosophy at some point
. Or not.