Burnt
Stan had spent nearly all eighteen years of his life in the frozen hell otherwise known as South Park, so when Kyle had invited him to come along with his family on vacation, he'd jumped at the opportunity.
Since it was the Broflovskis' last vacation together before their oldest son left for college, they'd wanted to do something special. They had originally wanted to go to Europe, but heaven forbid they travel overseas and bring their children closer to the war (one which Sheila vehemently insisted she had nothing to do with, lest anyone suspect her of being a warmonger), so that brought their choices down to either Canada, Alaska, Mexico, or South America. The former two were ruled out on account of Ike's refusal to go anywhere near his birthland again, and South America was ruled out due to Sheila being convinced that it was far too dangerous there for some reason or other.
So Mexico it was. By some strange strain of luck, probably due to the fact that they had sons rather than daughters, Kyle's parents seemed completely oblivious to how wild Cancun could be. That was the only logical explanation that either of them could come up with for their ending up in that tourist trap of a location.
But Stan wasn't complaining. He was just happy to have escaped the perpetual snow of his hometown for a couple months, and he was going to take full advantage of the sun's warmth for as long as he could. As soon as Stan and Kyle had settled into their hotel room, they'd checked in with Mr. and Mrs. Broflovski down the hall to tell them that they were going to the pool. Kyle's parents had insisted that they bring Ike and his friend who he'd brought along, but luckily they were content to stay in their room channel surfing for the time being.
Unfortunately, having lived in a mountain town for his whole life, Stan had severely underestimated the potential strength of the sun. Sure, he and Kyle had both covered themselves in sunscreen, but when you fall asleep in a lounge chair with the Caribbean sun beating down on your back for several hours... well, to not end up burnt would be a miracle. Even Kyle's Jewish skin hadn't prevented him from getting burnt, though he was significantly less pink than Stan - though that might have had more to do with him having a tendency to roll over in his sleep, so his burn was more evenly distributed. Stan's front was slightly burnt, but his back had become a peeling source of pain.
When Sheila and Gerald had found out, they'd lectured the boys for at least half an hour on the dangers of falling asleep in the sun and sun poisoning, which Stan may very well have gotten himself. As a result, the two were grounded - as much as a couple of eighteen-year-olds could be grounded while on vacation with minimal supervision - and told to stay inside the hotel for the rest of the week.
Neither of them had any intention of staying cooped up for that long, not with adventures just waiting to happen beyond the hotel walls. But as soon as Stan started puking and screaming every time he accidentally rolled onto his back, they realized that maybe it wouldn't be such a bad idea to stay put for a while.
Later that day, Kyle disappeared for a while. Stan would have probably been more irritated at his best friend for abandoning him if he wasn't nearly delirious from pain. But when Kyle returned with the purchases he'd made, Stan immediately forgave him for leaving.
After all, what says love better than noxema and a bottle of tequila?