Most of these are tips rather than hacks/stuff to take, but I hope they will be useful:
Alcudia has quite a lot of mosquitos, so take repellent in a bottle. It will be cheaper at Boots then when you arrive. If you want to avoid DEET (horrible smell, melts plastic) then there are several brands that use a different chemical called picaridin as their active ingredient, which is much less nasty. Your hotel may or may not provide a night-time mosquito plug-in. If it doesn't, you can get them in a convenience store for €5-6, or Lidl (there's one not too far from the beach) has one for about €2 and it even includes a little nightlight. You can get after-bite sticks at a local pharmacy if needed.
If you have bought a package holiday that includes a transfer, be prepared for that to take some time. Alcudia is only 70km from the airport and most of the way there is on a proper motorway, so the actual driving time is only about an hour. But if you are on a shared transfer it can take an hour to fill the coach up (while they wait for four other flights to come in), and then when you get to the resort it can take a while to visit every hotel to drop people off. You might be last on and first off the coach, but you might also be first on and last off. There's not a lot you can do about this apart from getting a private transfer (for 6 people this will be about €100 each way), but perhaps this can help you brace for it. It's frustrating to get off the plane and think you're nearly there, only to take more than 2 hours to actually get to the hotel.
Paying with a UK debit card in a foreign currency is a lot less expensive than a few years ago. Check your bank's web site for what they charge, but often it's less than 2%, and they have mostly got rid of the minimum fee, which could add 80p or so to every purchase.
You can pay by card almost everywhere but it's a good idea to have some Euros in cash. Either order them from your UK bank in advance, or get them out of a cashpoint that is attached to a proper bank when you get there. Do not change or withdraw money at the airport as the bureaux de change there are all licenced bandits, and do not use standalone ATMs in the street for the same reason. (Changing sterling cash into Euros at one of the change places in the town will often be not too bad.)
If you are paying in a restaurant by card and the machine offers you the choice of paying in Euros or pounds, always choose Euros. Your UK bank will give you a much better rate for the conversion from Euros to pounds than the card machine will.
You don't have to leave a percentage tip in a restaurant. Many Europeans leave no tip at all, and the card machine will probably not have an "add tip?" function. Certainly don't feel the need to add 10%. If your bill is €88 and you leave €3 in coins the staff will not come and shout at you. If you want to tip someone, leave €10 for the chambermaid if your room was kept nice.
Local buses are good and cheap. Pay by tapping your debit card on the reader when you get on and when you get off, like you would on the Tube, as it's cheaper than buying a ticket and you don't have to spend the driver's valuable time. You can put multiple people on one card.
When you arrive you will have to pay the tourist tax when you check in to the hotel. This is on a sliding scale depending on the length of your stay but it will be roughly €2 per day per adult in a 3 place or €3 in a 4 place.
Everywhere is very safe but there are pickpockets around. Go out for the day with one card each and €50 in cash. Leave your passport at the hotel - in theory you are meant to carry it all times but nobody will stop you and fine you for not having it on you.
Security at Palma airport is always very quick, so don't feel the need to buy priority or add on an extra half hour, if you have any say in when you get there on the return trip.
Super-niche tip: Since 2018, Spanish airports have to sell a 500ml bottle of still water for €1 (now €1.10 with inflation). Palma airport has machines that mix the €1.10 bottles with another brand that is more expensive, so look carefully before pressing the button.