1. The thingy with the wossaname on P34-8UY.
As if he hadn't learned his lesson on the last planet where they got chased out a village with spears. Next time, John's getting Ronon to babysit. With rope, if necessary.
2. The door lock in the broken lab at the bottom of the furthest tower on the West Pier.
Because then John wouldn't have spent six hours locked in a room with an increasingly irritable astrophysicist. The wrestling match over the last power bar is going to stick in his mind for a while.
3. A gun.
Nowadays he carries it easily, like it's the most natural thing in the world. Okay, he's not a great shot, but with that kind of firepower against the average Pegasus weaponry, he doesn't need to be. And John would take it to be a good thing, except for how he sometimes catches that look in Rodney's eyes. With the whole 'saving each other's lives' thing, sometimes it's easy to forget that John's there to protect Rodney, not the other way around. It doesn't really matter that Rodney's good at it. He shouldn't have to be.
4. Braelian porridge.
Welcoming feast or no welcoming feast, watching someone eat green goo with their bare hands is not something John ever wants to see again.
5. The Hair.
But when Rodney panics, he's not too careful about where his hands end up. Half a bottle each of shampoo and shower gel later, John watches the last of the bloody water drain out of the shower stall. This time, they got lucky.