Bug management/Triage tasks
Triage a single project
Pick up a project from the list of projects that interests you, or which you know about, or are willing to learn, or happen to use. You can query for existing open reports in that project and take a look at issues that interest you, for example in order to mention that the problem still happens (if there has not been any update for a while in that report), or noting that they have been already fixed in the version you use (please mention the exact version that you use in a comment, also see the section on reproducing in the Triage Guide).
Triage MediaWiki extensions
Wikimedia Phabricator hosts many extensions that are developed by volunteers. Their maintainers are always happy about a helping hand. The steps explained in "Triage a single project" also apply here. You can explicitly search for bug reports in extensions by running a search after setting the "In All Projects" field to those extensions that you are interested in. You can find the list of MediaWiki extensions here.
Triage the latest incoming reports
Try to reproduce reports which are marked as "testme"
Many reported problems cannot be easily reproduced. These reports have been marked with the testme project. Try to reproduce them if possible, and add a comment either with instructions to reproduce or that you could not reproduce it and mention the version that you use and information about your setup.
Update reports that have not seen comments for a long time
Take a look at old bug reports which have not seen updates for two years. Try to reproduce them with the latest software version to find out whether the reports are still valid. If they have been fixed in the meantime, you can set their status to "resolved" and add a comment in which version or on which Wikimedia website you could not reproduce the problem anymore.
Update your own reports
Most of us have submitted bugs in the past and forgotten about them. Take a look at them and check whether they still apply in the latest software version. Go to the list of reports that you have created yourself - you can click "Edit Query..." to only show tasks that have the status "Open" and "Stalled".
Update reports for a specific browser
You could try to retest older tickets that reportedly only happened in a specific browser if you use that browser, maybe in a newer version of that browser. Please always mention which exact browser and version you run.
Sync upstream and downstream reports
This is a cumbersome task that asks for automation but as long as nothing is in place it needs to be done sometimes: Query for bug reports that are in the "upstream" project and check the task descriptions which should include a link to the upstream report. The list of upstream projects can also be helpful here.
Sometimes patches to fix a bug report get merged in Gerrit but the bug report remains open. After carefully reading the comments in the bug report and getting an impression of the status and making sure that really all patches have been merged or abandoned in Gerrit, you could ask the developers in the bug report if the bug report should still stay open: Open reports with the "Patch-For-Review" project set.