This trick will allow you to earn as much SP (and money) as you can through Daytime Activities and Day Quests during your idle time. However, this is useful only when you're not playing the game actively since the main step is deliberately letting time pass by. That said, do this only when you're doing something else; like watching movies, playing other games, reading, studying, etc.
Note: You can also use this method to quicken the days it takes for the Trade Merchant to return.
Important notes:
Execution:
After setting up your Daytime Activities and accepting a Day Quest, head out to town. After heading out, do nothing. Let 5 minutes pass at least and return to the hotel. The game will calculate this as a valid evening activity and will trigger your Daytime Activities and quest. Just rinse and repeat.
5 minutes is quite fast especially if you're pre-occupied with other stuff. Even if you didn't manually return, just wait for the 15-minute time limit to run out and you'll be transported back to the hotel automatically.