If on tenure track or tenured at a big university:
Go to your lab whenever you like.
If you're teaching a class, go teach it. If anything needs to be graded, a grad student will do it.
Do lots of paperwork related to your grants.
Do work in the lab, check the work of your own grad students.
Work on papers you are trying to get published.
Plan ahead, tell secretary if you need travel requests, etc to attend symposia. If you're presenting a poster, get it ready.
Look over honoraria offers to present topics at other universities and such.
Talk on the phone a lot to colleagues, plan to go to their labs and them to come to yours.
Do your office hours.
Help your grads with their dissertation projects. Talk with grads hoping to get you to be their major professor.
Go eat lunch when you like, go running, goof off with your fellow profs, the ones you can stand.
Sometimes you'll stay at your lab all night or most of the night, depending on what you're doing.
Be aware that an experimentalist is very handy...does machine work, wiring, programming, plumbing even, lots of troubleshooting with the machinery. Lots of tinkering around with the equipment. Lots of calls chasing down this or that part needed. Some salespeople will call.