Costco:
No. With the Walmart in the same plaza with reasonable prices and not requiring huge quantity purchases, why bother with Costco? Although see my note below for an even better alternative to Walmart.
Family Activities:
We did the owners update and received what i considered a very reasonable rate to Isla Mujeres' Garrafon Park. This included:
1) roundtrip ground transportation from Lagunamar to the ferry dock.
2) roundtrip ferry with hot breakfast on the oubound
3) admission into Garrafon Park which included:
a) activities such as wall climbing, zip lining, kayak & snorkel use
b) unlimited food and beverages at the park (including alcohol)
c) use of towels, beach chairs, hammocks, pool, bicylcles
4) swimming with the dolphins with Dolphin Discovery
It is listed as $139 on the website. I think we paid $75 per person after the "owners update discount"
EDIT: Closer to Lagunamar, right off the the main Cancun drag (Kakulkan), where it intersects with Boulevard Tulum, is a Shopping Center (Plaza Las Americas) with a much cleaner, equally reasonable Supermarket called Chedraui. I remember that in my last trip, I much preferred Chadraui and would avoid both Costco & Walmart in the future. I was told about Chedraui by Lagunamar's senior manager. She is right.