Hi Adam, Firstly there is no such thing as free publicity, you have to always pay for it with with time, effort, or simple cash there is a price so decide what you can budget is.
I would always recommend a little planning discipline be put into place before kicking off. So ask yourself what do you want to achieve? how will you measure the success and what can you spend on it? (be it time, money, networking etc) The best PR stunts that seem to be free have often had an awful lot of time spent creative thinking or planning.
My advice would be, be really clear on what you want people to do once they hear about your service and put yourself in their shoes. How would you need to be engaged in order to do what you are asking? What is the journey you want them to take? and what steps will you take to lead them to the outcome?
Come up with ideas and test them on people you trust (Family friends, ask them to be honest with their feedback) and you should come up with a great idea that you can execute for a small investment.
Secondly, when it comes to PR keep it topical, what are the current stories or things that are likely to come up? how can you become part of those? Create PR calendar that looks at up coming events, holidays, occasions and look how you can be part of those.