There are some things that prove you're a couple. Survive these tasks, and nothing can beat you. This is a small and incomplete list:
Go on a long distance road trip and get lost, but stay together. Really lost. Guildford to Bournemouth via Portsmouth in the middle of the night kind of lost, with a navigator that's not too sure which way is left and which is right, you haven't eaten yet, and people are expecting you. Alternatively, take two hours to do a twenty minute journey because you get hideously lost, knowing that people are waiting for you to start eating.
Decorate a room and live in it afterwards. Aside from the potential minefield of surviving the colour decision, this requires coordination, drop sheets, some modicum of style, basic wall covering skills, agreeing to forgo the fluorescent orange just this once, and not treading in the paint and tracking at around the house. When you do tread in the paint, try to hop to the bathroom.
Assemble some "self-assembly" furniture together and not kill each other. For instance, that wardrobe you got in Chicago in 2003 that's been sat in the hallway waiting for you to paint the other bedroom and then move into it because the ceiling in your current bedroom wouldn't allow the wardrobe to stand upright. Bonus marks for discovering at a crucial point that you've assembled the dratted thing wrong and have to disassemble it, which is infinitely harder than the putting together in the first place, and then reassemble it.
Fly out of the country and return intact. This requires navigating customs, flight attendants, headphones that don't work and the ghastly necessity of an in-flight toilet stop. Yucky. In one direction or another you will be caught in the mass of people trying to get into a country not their own and possibly get your fingerprints taken. If the fingerprint man says "it's never done that before," add at least a half hour to your journey time.
Move house, bonus points if it's an international move. Deciding what stays and what goes, how to get rid of the excess, what to ship and what not to ship, and discovering the top layer of wedding cake, now three years old, sticky and somewhat furry, and arguing about who gets to dispose of it, then panicking about whether throwing out part of the wedding cake dooms you to an early divorce. Thrills for everyone.
Any other suggestions?