Beekse Bergen in the Netherlands was great with my 2.5 year old and just turned 5 year old this summer. We stayed in the safari resort, there’s also a lake resort with a beach.
We drove (&went on the overnight ferry), stayed in a self catering lodge, there’s plenty to do on site, and the Netherlands is very family friendly.
We are planning to go back in a couple of years.
We also went away last year in the Uk, to a random countryside village with extended family. Again, it was self catering so we could work to our own routine, and we planned things to do ahead of time, trips to local leisure centres with splash pads, a local farm
park, a transport museum, that sort of thing.
On both trips we took a couple of favourite books for bedtimes, some colouring and sticker books, a couple of cars/figures and some play dough plus the favourite cuddlies so if we were all a bit tired, it was a bit too hot or raining we had some things we could do for an hour or two in the accommodation. We also took our iPad because I’m not that wholesome 😄
its a bit dull but I think planning and being organised (and not setting expectations too high!) is key for success.