just make the changes and save with the preview button (Not the save button). Then only logged in people will see the changes.

I guess in your shoes, I'd clone my current site and use the clone as a sandbox. Even though C5 lets you easily rollback page versions, keeping experimental changes off the live site (until you're ready to commit) is a little less nerve wracking than having changes in preview where they can accidentally be approved for public view.

The drawback to this approach is that you'd have to reproduce any wanted changes on your live site. But for content blocks, that's really just a matter of copy/paste.

You probably know this, but C5 sites are easy to clone---basically just copy/paste the files to somewhere else on the server, create a new db, and point the new config/site.php to your new db.

I use a copy of my site on a local stage for that kind of testing. Since I'm on a Mac, I use MAMP. Here are some helpful guides:

Move a site (to copy your site) -
http://www.concrete5.org/documentation/installation/moving_a_site...

concrete5 on MAMP -
http://www.concrete5.org/documentation/how-tos/designers/installing...

concrete5 on WAMP -
http://www.concrete5.org/documentation/how-tos/designers/installing...