looking to clean up any stuff that Ive collected inadvertently through surfing the web

FIRST, if you are having disk-space problems, doing this will NOT provide enough space to solve Disk space problems. Post back for different advice.

The answer to your question as asked:

• Restarting your Mac removes any cached Files or similar data pre-fetched to speed up a web site.

• Pawing through your Downloads folder, and throwing out anything you do not Immediately recognize eliminates the largest files of most of the rest. But this is NOT inadvertently downloaded, you had to authorize each download.

• For cookies and small bits of data stored by web sites, that will requires some time going and deleting from:

Safari > Preferences > Privacy > (Manage Website Data) ...

... and deleting on a site-by-site basis