as long as you're truly in public space, you're fine. in the case you might get some shots you'd like to use commercially (either as stock or to promote your own services), you'll want to take along some model releases, as well.
keep in mind, some "free to the public" events that take place in public spaces might not be a "public event". for example, if a private company is producing the event, rents out the space, cordons it off, and manages all aspects of the event, even though they might not be charging admission, that space is theirs for the duration and they can decide what can and can't be done. that they would have a problem with you would be rare, but as it could happen, you need to be aware of the possibility.
if it's going to be a large enough event with lots of great photo ops, you might consider setting up an online gallery ahead of time and getting cards made with a QR code that leads to the gallery address. even better if you have a representative photo (properly released, of course) that you can use as a background for the card. you might also consider putting somewhere on your website a disclaimer similar to this:
As I am an active photojournalist, almost all images on this site are taken in public, or in places where I have been invited to shoot. It is not my intention to publish any images that would cause concern for anyone in the images.
If you find yourself in an image on my website and you want that image removed, or altered in such a way as to remove you from the image, please contact me immediately.
On the other hand, if you would like to order prints, you can do that, also.
have fun, good luck!