How do I lock my MBP screen without locking my keychain?
If you have a screensaver you can lock that with your password so whenever screensaver tries to quit it needs a password. To set this up, first set up your screensaver if you don't have one. Then open System Preferences/Security and Privacy. Click the first tab in that window named General.  The 2nd line on this box allows you to set a password associated with the screensaver.