Let me first say that you are a brave soul to nonrev during spring break. I have done it twice - once to Hawaii and once to Florida. It's nerve-wracking to say the least. That said, if you are the party type, CUN will be great at spring break (providing you can get on and get back!). If you prefer a quiet place, you might want to rethink the destination. As to the hotel, I recommend booking a hotel in advance but make sure it is cancelable in case the flight loads change. I generally pick a place that will allow cancelation with a couple of days notice. If you can't find a place like that, you might want to find back up plans in case the flights are good but the hotels are full. Remember, this is spring break and the college kids are looking for hotels, too!