This post is for people that have unlocked the bonus, if you haven't yet, or if you don't care if you get spoiled, read on!

The bonus is unlocked when you reach between 60 or 70 achievements. You check by clicking on Achievements on the main menu, and the game will count your achievements as you go through them. It counts how many entries are in the achievements list, but multigame achievements like "Five Dinners", "Shutterbug" and "Downtown Nudity" have multiple entries, hence the 60 to 70 number.
The bonus is, as many speculated, guessed, or wished for... Naked Dating! The plot is an app for nudists to meet fellow nudists, and you meet Ariane on the app, just like the beginning of a regular game. You show up only wearing a towel, and then take off your towels as soon as you are inside.
There are a number of differences from regular dating, and there are no paths to follow (you could enable paths, but they won't work very well). Ariane will early on explain most of the differences (no early touching, no downtown content, wear towels in public, etc), and here are the internal rules if you are interested:
1. You are not given any options for touching before dinner. No kissing, no moving in close, no slow dancing. Fast dancing, going to the park, and swimming are still possible early activities.
2. There are only 4 dinners. Like all other downtown activities, the fancy restaurant is off limits. The sexy themed diner on the other hand is OK.
3. Most of the allowed dating activities on nude dates are the same as non-nude dates, just nude. There are however 4 unique activities that are different than normal dates: First is the above mentioned diner. Second is going for a drive and Ariane is able to drive. She will take you first to the gas station, and then (third) to the vista area for a naked photo shoot, which has a surprise ending upon completion. Fourth, is a new park photo shoot you can only reach with a lucky bottle spin.
4. Make out sessions are harder. Bottom line, it requires more foreplay to get anywhere. For example on a regular date couch make outs, Ariane will allow you to grab her breasts (clothed) after 3 kisses or one shoulder massage. Under nude rules, everything gets upped by one, so you need 4 kisses or two shoulder massages. Similar plus one rules apply to slow dances, kitchen make out with beer or wine, or hot tub make outs. All sex situations are possible they just require a little more patience to get there. One part of makeout sessions is easier: getting oral sex. Should get a positive response in situations where Ariane is ready.
5. There is a hidden 80th achievement for getting a score of 10 on a naked date.
Note:
If you unlock the bonus, but want to turn it off so you can use the help paths, just go to "Preferences" and where it says Bonus, click "Hide". If you want to turn the bonus back on, just go to click "Achievements" and click "Next Page" about 11 times until it rewards you the bonus again.