Here's a proposal for what to test if you're stuck for ideas:

• Near a new release: run through the list of bugs that we've supposedly fixed. You'll be able to find this on Trac, it will be linked to from the RC announcement email to testers.
• Not near a new release: verify tickets people have placed on trac. Leave a comment with your details.
• Use the software as you normally do, with plugins. Don't forget things like feeds, enclosures, links, users, etc.
• Report back with as much information as you have. Install dump-env and give us the output of that. Mention what works, what breaks, anything you've noticed. The more information the better.