There will sometimes be a Very Important Patron in your hotel lobby who has some sort of glowing aura. Whenever you tap on him, he will give you some money so try to accept his offers especially early on because they will be super useful and help you make a lot of upgrades in the game. If you want to quickly buy a new battery and then build a second double room, then all you should focus on early on in the game is getting the VIP's offers. Those will give you plenty of dollars to get by and if you happen to be low on money and can't possibly pay salaries and such, then make sure that you take his offers.