Glad I'm not the only one who does this haha. I don't really do it in public or anything, but when I think no one is around in my house I start pivoting around corners and walking imaginary opponents down in my hallways.
One kind of weird thing I like to do when I'm cooking is set up a chair angled so that the 2 front legs are in the same position as the 2 legs of an opponent would be in. Then I just practice moving to inside and outside angles on the chair. I'll do it a couple times just moving, then a couple times with strikes, then with feints, then I'll throw strikes when I take the angle and angle on the way out and get creative.
But anyway, I like to drill footwork as a warm up then make sure to emphasize it at various points during a workout while including it with my strikes. For example, if I'm gonna hit the heavy bag primarily for my footwork then I'll start by just moving in my stance around the bag both ways. Then I'll push the bag and let it swing and move with it. Then I'll start hitting it as I move with it and adjusting to the new directions. So I might hit it with a combination, follow it as it moves away and hit it again, then pivot either way as it swings back towards me to get an angle and hit it more. My favorite is to sidestep and pivot to my right as the bag swings at me, then smash a right kick into the bag as it swings back like an opponent turning.
If I'm hitting pads, I often do it specifically to drill footwork. I'll pick a combination that I want to develop the footwork of and have my partner react like a real opponent. For example, maybe I want to get good at using a left hook with a left sidestep to get an inside angle to fire my cross from. I'll have the pad guy first let me throw the combination a few times without doing anything. Then I'll have him start moving to my left (his right) and I'll throw the combination to cut him off and land. Once I feel comfortable with that, I'll have him move in multiple directions until he moves to my left and I step to cut him off while using that combination again.
Then there are situational footwork drills with partners. One good example is the pivoting off the ropes mentioned by 3pac above. Another drill would be to have a guy come forward at you constantly with straight punches while you have to keep sidestepping and turning him in both directions. Or you could drill walking someone into a corner using only your jab. There are tons of options and a lot of them are pretty fun.
Footwork is part of everything you do in standup fighting and drilling it separately can never hurt.