I think this is a specific problem with a specific solution--it wouldn't work or be necessary for the majority of people. But it worked so well for us and I'm so pleased I wanted to share this story in case it could give somebody an awesome idea.
We live in Honolulu, and our families all live on the east coast of the US mainland. We have little kids (5 and 7) and in order to catch up with everyone we're related to, we take a big trip to see both sets of parents/brothers/cousins every summer. This trip is grueling with little kids, and it's also expensive because of all the flights involved.
We usually book the trip in January, and this year at that time we were paying two mortgages while we tried to sell our first condo. We couldn't bring ourselves to pull the trigger on $8k of plane tickets (yes, we travel hack, but we've already used all the low-hanging fruit credit card bonuses) so we thought we'd have to skip the trip this year.
THEN an old friend of mine from Honolulu who now lives in Portland, OR sent out a mass email to a dozen of her friends, asking if her family could do a house-swap with another Honolulu family in July, because she needed to go to a wedding here with her family and she didn't want to pay for a hotel. AirBnBs have become rare and expensive on Oahu because they've passed new laws banning them in most areas on island.
We jumped on this. We've always wanted to explore the PNW, and my friend's house is big enough for us to host some family there.
In the end, we were able to swap houses and cars for 2.5 weeks. My mom came out to visit us for the first week, and my BIL/SIL came to visit us with their kids for the second week. So much easier and cheaper for them to come see us in Portland than in Honolulu...and it felt special because none of them had ever been there. We had a terrific time exploring Portland and its environs, and the trip only cost our family $1800 for four plane tickets (we booked using the Alaska airlines Costco gift cards that were $500 for $450). The family that stayed at our house borrowed our cars, too. They were thrilled with the arrangement as well.
I've researched a website called homeexchange.com, but I really think this kind of thing would work best with trusted friends. It takes some flexibility, and you have to be cool (as in--you can't be too fussy about your stuff, and you have to have the right attitude about it and not expect hotel-level quality).
We're so psyched about how it turned out that we're now putting out feelers with our other mainland friends about doing the same thing next year in a different location.