Tricky. How does DH feel? Is it possible he encouraged them to come?
I've been on holiday a lot with my in-laws, they're very hands on with the children and a great help, but I still feel awkward around them. I know I don't meet their expectations but I've also stopped seeking their approval or caring too much what they think of me. Instead I try to see them kindly, and focus on the lovely time my children have with their grandparents, the memories they'll grow up with.
I find it easier if we have our own space, eg a self contained apartment next to theirs or cottages.
Why do you think they will want to do everything with you? They may want to have their own holiday, go sight seeing or for some day-trips, then meet in the evening for a meal out? Or look after children some evenings so you and DH can go out by yourselves?
If they do come, maybe write a plan for your family of the things you've planned to do, like beach trips, and leave some days or afternoons free for doing things together?
Softplay in the rain actually sounds grim, what if it rains all week? Would having 2 extra adults allow you to try some new indoor activities? Explore some local indoor attractions?
We do things like interactive museums, boat trips, butterfly house, touring castles or guided tours of caves. When grandparents come they're often happy to play with the kids indoors for a morning while DH and I explore the places less suited to kids.
Is it possible your DH is getting fed up of only doing beach trips and softplay on the annual holiday?