When I (admin) go to Search > Find Activities, the criteria includes a selector for Campaigns. Authenticated users, however, do not get that option. See the snippet below from such a user, in my criteria the Campaigns selector is right below the Location box. Likely means they're missing a permission (Drupal 7.69, Civi 5.21.0), but don't know which. There are a number of permissions associated w. CiviCampaign and I'd rather not open all of them because our use case is very narrow. Any guidance on which permission to grant, if that's what the issue is, appreciated.
This is related to this bug 'CiviCampaign: access CiviCampaign' permission missing. Unfortunately at this time, in order to get Campaigns search showing in Search -> Find Activities, you have to grant permission to
Depending on your user base (trust and size) and how bad you need this working now, you could give
administer CiviCampaign and then change the menu permission for the Campaigns top nav menu item to
administer CiviCRM. They would only be able to get to those menu links if they knew the URL directly (unlikely).