There are a number of ways to increase your XP, cooking dishes and serving your restaurant guests will earn you XP to Level Up and Gold that can be used to buy decorations.

Harvesting Crops and buying animal Farms and making animal products also gives you XP.
You will also receive XP when you assist another player loading a crate on their Delivery Truck or with their Chillout Event, making a donation in your Chef Club and when you expand the terrain around your restaurant.  

Each new Level you reach you may unlock new chefs, tables, decorations and you may receive also receive special rewards.