Probably the best advice is to quit reading this board until you get back. If you keep looking at everybody's gripes, complaints and niggling worries about whether room 216 is better than room 218, and whether formal night is Monday night or Tuesday night, and whether they need to pack hair dryers, duct tape, power strips, cable ties, elastic cords, soap, food, water, etc., it's sure to make you nervous.
For a short cruise, pack light and sensibly, follow the suggestions in the cruise information, get on the ship, go with the flow, and have a good time. You can't go wrong because cruising is the most stressless way to vacation. You make comparitively few decisions of any importance, everything is done for you, and you can turn off your brain if you want to (of course, some cruisers take this last point to extremes!)
Forget about it until it's time to pack (a day or two before you leave), then go and have fun. You'll love Tulum, it's a great choice for an excursion.