Marketing is a good way to get more clients. You can use it by researching "Marketing spots". This gives you access to two options.
The first option are marketing spots. They are scattered around an entire city. Renting them allows you to encourage more people to visit your restaurant. Their price is the same which means that you have to choose them wisely. They work on all social groups which gives you many possibilities.
Marketing spots have a certain range.
Marketing spots that are near places visited by tourists or public use buildings are the most profitable. Areas near metro stations are also good. In this case your income and number of clients can depend on how many people was in range of a given column.
Your second option is to run an advertising campaign through mass media. Their effectiveness depends on how much money you want to spend on each of the types of mass media.
Also, a given type of mass media can reach different social groups, e.g. place ads in newspapers to attract attention of old people, social media won't work on them. This marketing method works great when you want to focus on a particular group.
Hiring a mascot is a very good idea. You can do that by opening the menu of a given restaurant. Select the advertisement option. A mascot at level 1 costs 1,000$. This level can be increased twice. Notice that a mascot with a higher level costs more. However, its level also increases its area of effect and efficiency. A mascot has specific working hours that can be adjusted with the slider. It can also move to places pointed by you - thanks to that you can choose places with the biggest traffic.