Choose a simple one. Security is not a big issue here because even if its cracked its still going to be very difficult to get at your banking details. All we are trying to do is make it difficult for somebody to use your network. Secondly smart phone users have poor keyboards so its difficult enough to enter a key if its a mix of letters and numbers. Stick to something simple like a few letters followed by a few numbers. Even abcde12345 will be enough to dissuade the casual free loader and its straight list of letters and numbers are easy for the smart phone friend or relative to enter.
By the way it’s encryption not just a password. So its not just a means of entry but every bit of data you send is scrabbled by the encryption key – very difficult to make sense of, so use one and feel safe.NB For owners of cafes providing free WiFi, you should speak to me – your requirement is different.