Take a walk along the beach… with blanket in tow
If teenagers can do it, why can't their parents? I'm not saying you have to go all From Here to Eternity with this, but you can do your own private version of it for sure. If you have older kids, it's easy to sneak away for a bit with a blanket and some courage on a dark night. Find a secluded spot, throw down your blanket, and get to it. If it helps, bring a bottle of liquid courage and two plastic cups to help light that fire.
Skip the skinny dipping
One place not to get your groove on, no matter how great it might sound, is the hotel swimming pool. To wit: my friend Rachel, who was was sharing a hotel room with her husband and three children. After the pool closed at 9pm, she and her husband snuck over to it in the dark. As they disrobed and slid into the hot tub, the front-desk clerk came barging in, threw on all of the lights, and told them to get out. Best of all, the pool room walls were floor-to-ceiling windows; anyone wandering by could see them getting dressed for their walk of shame.
But, bright side, another 15 minutes and they'd have been golden. Don't give up on vacation sex! This is your time to relax and enjoy life. You just have to get creative and pack a few essentials to make it happen. Just, seriously, trust me on that "Do Not Disturb" sign.