The internet has lots of sites on which to promote city activities and events. The majority of the more credible ones are not free and that would range from prominent news outlets to the tourist industry websites. The sites would certainly guarantee some degree of traffic to your event.
Then on the other hand, you have free classifieds like Craigslist for the US, the more less known site that is still generating some buzz Fonolive(US and Canada), kijiji for Canada, EventBrite, Facebook and twitter. The list trails from there.
Craigslist is great in the US although its more popular sections are , well not surprisingly the personals. You certainly do not want to post your event or activity in any of the categories in personals. It will quickly get flagged and deleted no sooner than it got posted. Unless your event is directly related to it like a swing party or something.
EventBrite has created a pretty strong stand already in the events category as it is focused mainly on events. Its main advantage is that you can directly sell tickets on it. EventBrite seems to be left behind when it comes to people who haven't heard about it or thought about going to an event or not. These are the people that are most difficult to attract. This is where sites like craigslist and kijiji thrive as they offer a vast array of categories and someone could just wonder to the events or even personal category just to see what is going on there.
Facebook and twitter are great if you already have a huge number of followers otherwise there is very little chance anyone would be able to find your activity. That means, if you don't yet have any friends or followers, to successfully promote events or activities, you must start building that following several months in advance. That's a pretty daunting task for an event planning.
Then you have Fonolive.com, still little known although it building traffic somewhat significantly judging by its growth in the Alexa rankings. Fonolive.com is yet to have an option where you can directly sell tickets. The last time I checked, they seemed to be working on that. Its main advantage is solely that it is focused on what is fun in every city from events, activities, city attractions to restaurants. Interestingly, it also has a fairly robust social network within the platform where its users can friend each other, share photos and exchange messages. That seems to what is generating the buzz and traffic for them. If you are an event planner, this is definitely the place to post events as you are directly connected with other users in the city.
For every event planner like myself, the plan is always to throw the net as wide as you can and see what kind of fish you catch. Catching many tiny fishes is better than catching a few large ones, but that also depends on what kind of event you are promoting. Good luck with your event promotion!!