Cicopo already gave you some good info there. The beaches closer to town on the south side tend to be rocky past the waterline. I'm not sure about the resorts north of town as I have done only one Surface Interval at one up there, but it did have sand out into the water in front of the hotel. Northern hotels are a different deal and many of the shopping sites give inaccurate locations so check the actual addresses for "Norte" and look at the hotel site before booking thru a discount site.
Since you mentioned
"trying to keep the costs as low as we can, without sacrificing too much," I played with some ideas...
Depending on how and exactly when you want to fly, you can leave SEA around 6am and arrive Cancun around 5:30pm, but still have to do the bus/van & ferry connection. I've done it and still made the 8am boat but it's not for everyone. Flying from Cozumel (CZM) to SEA the same day is easier with the time change: Air would be $450 & up each RT, depending on how but I would certainly try to return CZM-SEA and not do the ferry & bus/van on return.
I think you might prefer the overnight flight flying into and out of Coz: SEA @ 11:50p, 3 hour connection, to CZM @ 11:17a with various choices on same day return for $474 & up.
Now that is high season, altho at least it's not Xmas or February's Carnival. Looking at air & hotel packages, looks like you can get the Allegro AI for $992 ea including air, hotel & taxes = $148/night for hotel portion taxes included. For the Wyndham, Grand & the Iberostar it looks like $1265-1290 ea = $226/night or more - which sounds high even tho they are top end resorts.
There is an Orbitz discount code for taking $200 off of any package but it currently has a glitch. The code is
200entertain is you want to try it, or phone Orbitz.
If I use the same dates booking the hotel separate from the air, Allegro is only $134/night including taxes, but the other three have a wide range of Iberostar @ $161, Grand @ $185, and Wyndham @ $288!
I am not sure if those prices are all AI, but
if they are, some may be better than the packages.
If you do get a package tho, do not buy the ground transfers option as you can handle that easier for half price there. I wouldn't book any options in advance.
I've never been to Isla Cancun, just into town once, and I don't know much about Playa del Carmen-Tulum area other than the diving is best at Coz. If you're wife wants to relax on the beach, drink, swim in the pool, maybe a spa day - she might like the southern resorts, and there some tours. If she would want to buy a second bag and fill it with shopping, she might be bored down south and round trip taxis to town might kill your budget.
