I've been to the Cancun area and to several other resorts in Mexico. I suggest you bring the D90 and one zoom lens (18-200, since you have it). I have an Eagle Creek
padded 'fanny pack' that has has a shoulder strap and the waist straps that can be stowed. No model number but it's approximately 10Lx6Hx5D. The D90 with my 18-105 lens drops right in while the 'shade' is attached. That combo will take 98% of the photos you want while 'in the field'. Bringing along more lenses is asking for salt, sand, and more trouble.
A bag like the above doesn't scream EXPENSIVE CAMERA INSIDE and you can bring it nearly everywhere.
I've had no issues with the resorts' room safes.
Bring extra memory cards. Take large/normal JPEGs but add RAW when the the light is mixed.
Take some panoramic shots from your balcony, beach, etc. Handheld is okay and I recommend holding the camera in the 'portrait' orientation with the horizon in the middle of the frame. Practice locking the focus and exposure. Example created with Photoshop Elements:
If you have an SB-600 or similar, bring it for some night shots and some fill/bounce flash. The resort will probably have some evening shows with dancers, etc. Practice some slow-shutter techniques with the flash set on 'rear curtain'.
If you see another couple with an SLR, offer to swap cameras and take each others' picture(s). Your photo album will appreciate it. Caution: I encourage you to set your D90 on 'Automatic' (a.k.a. "waiter mode") for these shots. I have a bunch of shots where I had the camera set for the photos I've been taking and not set it properly for a photo of me/us. (e.g. center focus point which then focuses between the two of us.)
If you go on a sailboat, party boat, snorkel boat, etc., DO NOT bring the D90. If you have a point & shoot camera, put it into a couple of zip-lock bags and bring it along instead. You will have a MUCH more comfortable boat trip.
Enjoy your trip!