Frank
To unlock Frank at Level 2 you need to get the Laboratory.
Dennis
To get Dennis staying in your Hotel you must unlock the Jump Tower and complete some quests.
Tesla Lamp
If you buy the energy efficient Tesla Lamp you will be able to light up your Hotel.
Tombstones
Decorate your Hotel with Tombstones as plenty of guests are fond of them.
Fence
The best way to keep unwanted things off your property is to use the Fence.
Long Tasks
Remember to set up your long tasks that take 24 hours or longer to complete at the end when you finish your game so they will be completed the next day you return.