Here is a guide on how to get secret achievements in Planet Coaster.
Planet Coaster Secret Achievements Guide
1. place down Monseuir Frites and configure it from the fries and put alot of salt on it.
2. Go-Karts. In the ‘Track Rides’ tab, you can find a simple go-kart track from a blueprint (or create your own)… **DLC’S WORK TOO!** Rename the ride to ‘bollard’ wait for a queue to form and wait in the seat position. When the race starts, use WASD to drive. You NEED to get 1st place in order for it to work.
3. Create a crash involving 300 guests, for e.g, create a rollercoaster at about 125+ mph and crash it down on 300 guests.
4. Fire a staff member. All you have to do is hire a staff member then fire it. Easy right?