Our family trip was to Harbert, Michigan this summer. We survived a long car ride with the kids, including about an hour of Marlo crying "Mommy now!" from her car seat. Still, it was a lot less stressful than flying.
We rented a house with Aunt Rachel, Uncle Jeff, Cousin Kinsey, and Grandma and Grandpa. Plenty of space for kidlets to run around, and even better...a pool. Hayden and Marlo were in the water pretty much any chance they got--the pool every day, and Lake Michigan on other days.
The private beach nearby also had a small little stream which was Marlo-sized. Marlo gravitates toward all puddles and mud, so it was perfect entertainment.
Hayden was mostly interested in playing in the big lake. He liked being tossed around by the waves. Occasionally, he would get out long enough to find some rocks or mess around in the sand, but mostly he wanted to be in the water.
We also went to a Go Kart track one day. Hayden loved it, and Marlo, always an adventurer, wanted to try as well. Her enthusiasm waned a little bit once she was in the car, though.
The kids collapsed into bed every night, pretty much exhausted by fun. I followed soon after...