I think doing "special" things is lovely but the things that bring the long lasting joy is learning how ordinary life works for your family overseas.
I loved learning about the day-to-day activities, the lay-out of their home, their neighbourhood and their lives. I like to see the places they call "home".
When I lived in America with my little ones, and my parents were in the UK they wanted to know about my children's favourite parks, the supermarket we walked to, the "ordinary" things. I found this peculiar at the time but indulged them with random photos and sketch maps by post. They couldn't visit us all those years ago. It was a surprise to me, years later that these were exactly the things we wanted to know when our son moved the other side of the globe and our daughter and family moved abroad.
Yes, just "hanging out" with your overseas family is THE BEST.
Treats are lovely, but shopping and walking, collecting little ones from school (or taking them to a playgroup) - sort of, walking in their footsteps, or at least alongside their day-to-day lives, is what I bring home these days.
Also, maybe build in a few days when you don't hang out with them. Have some suggestions for places to visit. I think you will both need days off!
Have a lovely visit.