-When your park is successful and thriving, put up a "No Entry" banner in front of the exit. A significant amount of guests who want to leave will not be able to, and will remain near the exit. Note: Your park rating will drop. When you remove the banner, the crowd will stream out.
-To keep guests trapped without using space, first pick a guest that you want trapped in the park. When the window appears for what to do with them, pick them up. Instead of dropping them, just close the window to have them trapped forever without ever seeing them. This also stops them from getting happier/unhappy, hungry, thirsty, needing to use the washroom, or tiring out. This is best used when the person you have selected is very happy. If you are nervous about a goal requiring a specific number of people, you can use this to grab as many as needed. Alternately, you can empty a park while keeping ratings and number of people.