I'm going to Spain tomorrow!
Everyone should have at least 6 months on their passports (presumably you all have new ones so no issue there)
Travel insurance
GHIC card ideally (but don't panic if you don't have them)
Bank cards
A few euros
Airport parking/taxi booked/public transport planned.
Allow loads of time to check in. Better to be early and hang around at the airport than cut it fine and be flustered.
Travel once you arrive in Spain planned.
Accommodation confirmed.
Any medication (ideally take a copy of your prescription, a medical letter or a print out from your surgery for prescription meds, especially any restricted painkillers or anything like that).
That's it for essentials I think.
Pack (some of this might not be relevant depending what you're doing):
Swimwear
Towels
Arm bands/inflatables/goggles/snorkels/sea shoes
Underwear
Socks
Trainers
Sandals/flip flops
Shorts
T shirts
A cardigan or jacket
A couple of smarter outfits for evenings
Something modest if visiting churches, cathedrals etc
Sun hat
Sun glasses
Toiletries, make up, hair brush, sun cream
Devices and chargers
Books
If you forget anything, don't panic! There are shops at the airport and in Spain.
Have fun!