Not going to lie, this is the most challenging age to take them away. They are on the move but can’t be reasoned with. They don’t sit still on the plane, and with naps, you can get stuck in a hotel room most afternoons. Good shout to take the in-laws. Also, they can’t walk too far, so don’t plan too many excursions.
Take a cheap stroller rather than a large buggy.
Take lots of changes of clothes on the flight in case of air sickness, plus carrier bags and wipes.
Convince your oh to have dc on HIS lap 
Careful with balconies and pools. Someone must always be on duty to watch them like a hawk!
Take arm bands for swimming, plus swim pants.
Take one or two baby books, maybe some crayons and colouring book. One teddy. Don’t bother with toys except maybe bucket and spade. There will be plenty to keep her occupied.
Make sure there is a crib at the hotel and take a small blanket just in case.
Take all your own nappies, wipes, nappy bags.
I echo taking a drink and dummy in the flight, plus snacks.
It will be fab! If at times stressful. do ask your in-laws to have her for an evening so you and your partner can have a night off.
Enjoy the sunshine!