Edit: Most common responses seem to be:

(To clarify, these are a general summary of what other people have been saying in the thread, not my own opinions on them)

• Sex in water (ocean, hot tub, swimming pool, etc.) - Water washes away lubrication, and makes it harder to thrust, salt and chlorine can burn
• Sex in a car - Cramped
• 69'ing - Too hard to give and receive at once, awkward positioning
• Road head - Way too distracting while you're trying to drive
• Anal - Messy, smelly, doesn't feel that amazing, too much effort to do successfully
• Threesomes - Awkward, one person gets left out, messes with relationships
• Playing with chocolate sauce, whipped cream, etc. - Sticky, can only eat so much before it gets gross
• Sex in various outdoor areas - Uncomfortable surfaces, bugs, dirt and sand gets into unwanted places, splinters
• Titty fucking - Just doesn't feel all that special, breasts are too smooth to provide much sensation
• Blowjob with Poprocks - Just doesn't feel good
• Sex on various drugs - Makes the experience strange rather than enhancing it
• Candle wax - Burning is straight out painful, not light and erotic painful

Not that shower sex doesn't work, just that I found it to be more awkward and distracting than having sex in other places. Most feasible position seemed to be doggy style standing up, which was awkward with the girl in question since our heights didn't line up well. The heat of the water dried her and the condom out a bit, so it was hard to get it in. You can't have sex under the water, since it's distracting, but if you're away from the water then you're just having so so standing sex in a bathtub. Did it for maybe a minute before we decided to towel off and go back to the bedroom.