Back in April, 2007 we had some fun with Secrets you might as well share now and I was tempted to risk a bump to that thread, but the zombie fear made me choose this route. Besides, I want to focus on those things you say or do when you know you’re alone and nobody else can see or hear you.
One of those things I do in the car is to make up ridiculous names for people and say them aloud. Creative cussing is another outlet.
Yes, these are tame enough I shouldn’t feel embarrased to mention them, but I often wonder if the car were bugged and somebody played what I was saying on some website like YouTube would I be mortified or worse.
How about you?
BTW, bump that old thread if you dare! 
Read scary stories aloud. This could be from a book (like Stephen King) or from the internet (like creepypastas or SCP foundation pages). Sometimes I’ll also read articles and such that involve a lot of emotion so I can get all fired up.
I’ll also sing or take out my violin and play it. The reason I never practiced outside of school or lessons was because there was always someone who could hear me–if I’d had a soundproof booth or something when I was growing up I’d be a professional violinist by now. Or an opera singer.
I play out dialogue from scenes that pop into my head for various writing projects, mimicking voices and assigning accents to the characters along the way. It’s not so much talking to myself as facilitating conversation between the voices in my head. 
I often sing entire Kesha songs from memory.
Recite song lyrics with all of the "you"s replaced with "thou"s and other such silly anachronistic words.
Sing. LOUDLY.
Dance. I also do this in front of other people of course, but it’s different.
Talk to the cats.
Talk to myself.
Masturbate, duh. 
I’ll deny it if asked, but there may be lots of burping and farting.
Actually, I only need to wear noise cancelling headphones
[spoiler] to masturbate,
[spoiler] on the bus.
[spoiler]Because people are always yelling at me. And it’s always the same thing.
[spoiler]“What’s the matter with you? You’re supposed to be an adult!”
[spoiler]And I always say “Relax lady!”
[spoiler]And she’ll say, “But you’re the driver!”
And I’ll say “Look lady, chill out. I’m going to get your kid to school on time!”[/spoiler][/spoiler][/spoiler][/spoiler][/spoiler][/spoiler]
Sing to my heart’s content.
used to do it freely when people weren’t around, now I’ve gotta boot the pets too.
African Grey mimics. Severa macaw objects. Cat can’t stand all 3 of us.
Sing oddly. Make weird noises and faces. Whoop
I love that so many people sing. I sing all the time, but I only sing the songs I’ve written when I’m alone. I also talk to myself - mostly angry responses to someone else that I refrained from saying at the time.
I used to amuse myself in the car ride home by practicing my burping.
Sing/rap. It’s amazing to me how I’ll be such an expert singer/rapper when I’m alone, but if I ever attempt to do it in front of someone (and my wife is about the only person who would be privvy), I find that I know far fewer lyrics to the song than I’d thought previously. I don’t notice those gaps so much when I’m by myself.
Watch really, really corny old musicals, and sing along with the music.
Laugh.
My mother takes exception to my laughing at things like The Big Bang Theory, Family Guy, American Dad!, funny pictures of cats on teh interwebs… so when she’s away in Respite, or at Daycare, I watch these shows/look at these sites and laugh myself sick at them.
Like a boss
Talk to myself. Constantly. And, if I’m playing a video game (like Skyrim, especially), I have elaborate conversations with the NPCs my character is talking to.
Shopkeeper in Whiterun: Let me know if you see anything you like.
Me: I like how, whenever I walk into your shop, my minion comes running out of your bedroom.