Help Creating Report Filter

I'm trying to analyze activity data for a number of contacts in our database. To give a bit of background, we ran an advertising campaign and asked interested users to submit their information. From the submissions, we created leads - a percentage of these leads were converted. Of those that were converted, there was a decent number of contacts who were already in our system.

I am hoping to design a formula that will allow me to pull activities after the contact was associated to the campaign. Is this possible? I don't simply want to pull an activity based on the modified date in case the contact had basic contact information augmented since the individual was added to the campaign.