Whats the revenue model here? Do you charge for access? Do you give it away for free? If its the latter, would you be arrested if someone abused your service? Are there people/services available who would help you set up such a service?
While it is true that offering Wi-Fi by itself is not the way to making your millions, you can make a start by offering Wi-Fi in a controlled environment. This means that you control: -

All of the above is available as a managed service offered by companies like Pronto Networks, Airpath, Aptilo, Nomadix and so many others.

When deploying outdoor wireless mesh solutions, we've found that obstructions close to the mesh router antennas will significantly impact the propagation or radio waves. For example, trees and edges of buildings within about 20 feet of the antenna can degrade range by as much as 50 percent. So, be careful when planning the installation location of mesh routers!

In most cases, the downstream signals from access point (measured at the wireless client) will be higher than the upstream signals (measured at the access point) because the radio cards and antennas in wireless clients generally offer lower radiated transmit power. The client radios usually have lower gain antennas. As a result, measuring only the downstream signal strength may offer coverage that's better than what applies to typical wireless clients. In order to correct for this, you can perform RF surveys by only measuring the uplink signal strength. You can log into the access point statistics page, though, and read the signal strength of your wireless client. Most survey tools only support downstream measurements. Alternately, you can measure the downstream signal strength and take into account the lower radiated power of the applicable radio card making the measurements.

I've been doing some testing with Cisco 7920 wireless IP phones, and I've found that it's very important to equalize the transmit power on the phone and access point. If this is not done, then you'll likely have areas within the facility where audio only goes one direction. For example if the phone is set to 20mW and the access point is set to 100mW, then there will be areas where a connection is possible, but transmissions from the phone to the access point may not make it through (but they will in the opposite direction). As a result, be sure to set the phones and access points to the same transmit powers.