At this age in the past children would have had much more freedon to hang out with other children and really not see their parents for hours on end during the summer holidays. Maybe they are hankering after some freedom to experiment and make their own decisions about what to do. Not really practical I know in this day and age.
I'm not sure what the answer is but it is definitelly not 'more exciting' or expensive trips.
My niece and nephew who were brought up by their dad after my sister died - got no almost trips/outings from their dad at all. They besically went with him where he wanted to go. He isn't mean at all and the kids love him, he just has has NO interest in doing kiddy type stuff and doesn't see the point.
You couldn't hope to meet more delightful kids who were genuinely enthusiastic and grateful when they visted us in England and we took them out. I have almost never heard them say that they were bored and this is in 8 years of helping to look after them for 5 weeks every summer and 2 weeks at Christmas.
They loved museum trips and still do at 13 and 15 yrs old. I took them to Chessington one year and they were absolutely amazed that there were kids there on a school visit. They get almost no school visits in Italy and what they do get are certainly not to theme parks.
This summer they spent 3 weeks with their 80 year old grandparfents and never once said their were bored either. If I asked them if they were a bit bored, they said 'not with being with nonno/nonna but I do miss my friends a bit'. That was the extent of their complaining at being stuck with 80 plus year old grandparents for 3 weeks as teenagers.
I vivdly remember taking dns to the theatre one Christmas with my cousin and her daughter. My niece was horrified when my cousin's daughter complained that her mother had bought cheap seats (she was 8 years old)!! - She was delighted to be going to the theatre at all and couldn't belive that ther other little girl was complaining about the seat.
Often the more material stuff you have the less it satisfies