Below you'll find 3 ideas to fool your colleagues.
Secretly work together with someone from HR and make sure that someone is added to the group chat of the company. He/she will be introduced as a 'new employee', with an unknown phone number (for example an old number of yourself). Then send awkward messages from that number and see how everyone reacts. Laughter guaranteed.
Make up a reason to treat. Your colleagues' mouths will be watering when they see the Oreo cookies that you are sharing. They just don't know yet that you replaced the white layer with toothpaste. Pay special attention to their facial expressions when they take the first bite.
The moment your colleague goes to the toilet, quickly lower the speed of their mouse on their computer. Once your colleague is back, the confusion can probably be seen on his/her face and you can enjoy watching a few minutes frustration with his/her mouse.