We're just back from an all-inclusive beach trip. I used a LandsEnd string back back for the beach - only needed sunscreen, phone, kindle and sun shirt.
As for sunscreen and bug spray, we took travel sized ones as part of liquids to get started. I eventually bought a sunscreen at the resort which was of course outrageously overpriced, but we only do carryons so I was expecting it. We didn't use the bug spray at all, basically saw no bugs - and DH is super paranoid about potential long-term problems with Zika for DDs in the future. You could also look into getting the bug spray wipes (we got Cutter ones, Amazon has them) so then it's not a liquid, I just had them in my suitcase.
Dress was only "formal" at a couple of dinners, I did sundresses and sandals, DH did nicer shorts and button down with Tevas. You're still on vacation after all but not sure how strict your resort will be. The rest of the time it was shorts/tshirts and flip flops. I way overpacked.
I saw a lot of people with stainless mugs, I admit I was jealous of their continually cold drinks! Also saw some people had towel clips on the lounge chairs to keep the towel in place.
The beach lounge chairs were "reserved" early, some days we couldn't find any "good" ones after 7am! So be aware it might be hard to get seating especially if they have coverage or shade. We finally learned to keep the pool towels overnight and throw those on the loungers we wanted early along with a cheap pair of flip flops. We were usually there by 9-10am but saw ones around us go completely unused until the aftn which was annoying.
Also, plan on a way to keep your cellphone and room key separate. We must have gone through at least 6 keys getting zapped from the phone and then not working. It was hard bc I'm used to throwing all that in my purse but had to figure out something else each time we'd go out!
Have fun!!
"Every mother needs a wife." - Amy Poehler, Yes Please