I am not sure what my answer would be, but I know unequivocally what my husband would answer - a bass fishing excursion. He did one last year for the first time, just him and the guide on Bay Lake, and he says it was worth every penny and wants to make sure he does it again on any/all future trips, if possible. The guide was knowledgeable, the experience was serene, and the fishing was great.
My husband and my nieces did this and they loved it. They treated DH and actually only invited him, so it was extra special.
Keys to the Kingdom tour was an incredible experience.
My family did this one and found it to be money well sent. My DGD was too young so I stayed behind with her while they went. I am still hearing tidbits of info they were told so many years ago.
My biggest splurge is I think discontinued. It was called the Sunset Savannah tour at AKL, a 1 1/2 hour tour of the savannas of both Jambo and Kidani followed by Family Style dinner at Jiko's. Come to think of it, it's been so long that the first time I did it Kidani was being built, so we only toured the Jambo savanna. Less costly was "Lunch With An Animal Keeper", which I also loved, it was held in a private alcove at Sanaa, also Family Style, with an animal keeper from the resort. That too has been discontinued, but the last CM I spoke with said that it "may" come back.
We did the Sunrise Savannah tour that you could only book if you stayed CL at AKL. It was 19 years ago I think, and still rates the best money we ever spent.
My family tries to schedule one or two "extras" every trip. We tend to stay deluxe because the resort is part of our vacation, but we used to book CL once in a while. We liked the additional amenities. I don't think I can fund the value vs the cost lately, so that one is out.
We also book a spa day for the girls once in a while. I do enjoy a massage and the spas are so lovely in WDW.
The parties are one of the extras we have always enjoyed. SO far we have never had buyers remorse. When my DGD was young and loved the characters, we did consider booking 2 in a trip, but never ended up doing that.
Behind the Seeds. Excellent!
Personally, I think that the extras can be lovely if you can add them comfortably into your budget, but I consider that :making the moment" is the best extra ever. I know that park tix are expensive and it can be tempting to drag every cent out of each one, but I think that where meltdowns or disappoint will be on deck. The parks are magical, and the details can either be a distraction or a plus, depending on your mindset. I learned that lesson on our first real day in the MK when my DGD was 4. My Sis IL and I were rushing to the next attraction, and my DD and DSIL were lagging behind with Kady, so I tried to rush them. My DH took me aside and pretty much threatened to haul my bossy self back to the resort if I did not stop that immediately. He was so right. Spend what you can afford without resenting it if you take the time to enjoy the moments you are in rather than racing to find the next one.