In his spare moments, he likes to practice Force-assisted, single-handed ball-juggling (as he dabbled in that in AOTC, when he was showing off to Padme).
Vader goes scuba diving. Just listen to him, and it makes sense.
A few years ago when I was still in media sci-fi fandom, I had an idea to do a costume of Darth Vader in scuba gear. It wouldn’t be that hard – dive gear is already black, Vader’s life-support breath sounds come from a scuba regulator, and I could cheat with a prop tank. I would have had to make the chest panel, of course.
after looking at the site hail ants links to, i may not know what darth vader does with his spare time; but i know what a whole slew of people do with their spare time.
I was amused to discover that that site also contains a detailed explanation of why the explosion of the second Death Star would have made the Endor moon uninhabitable and resulted in the extinction of all Ewoks. 
Baked twice by means of those purple dark-side-of-the-force lightning bolts. (I’m sure the Emperor has taught him how to do that trick by now.)
Strangling wire.
What does he do in his spare time?
Plays with Star Wars action figures (oh wait, that’s Dark Helmet)
Does he take his gloves off to wipe?
No, he just uses the force.
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On his days off he does voicework for James Earl Jones. Most people think that Jones did the voice of Vader, but this is backwards. In real life Jones’ voice sounds like that of Rosie Perez.
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All Star Destroyers must be Vader friendly thanks to Federation Rule 21A-6 pertaining to the Galactic Peoples with Disabilities Act.
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Vader may spend most of his time on his Star Destroyer, but his mailing address, oddly enough, is a house of ill repute on Kashyyk. Vader loves him some Wookie love.
You are an evil, evil little man.
sets up stall
Sign reads Brain Bleach: $10 a bottle
This adds a whole new dimension to the end of ESB when Vader tells Lando to bring the princess and the wookie to his ship.
[Eddie Murphy channeling Mr. T]
Hey Chewbacca boy - c’mere! You look mighty nice in them bandaleros n’ fur!
[/Eddie Murphy channeling Mr. T]
O waiter!? One order of brain bleach - to go, thanks…
Ironically close to the truth. JEJ has a bad stutter with his natural speaking voice, but like many stutterers, he doesn’t have a problem when using an affected voice. He started using The Voice just to get around the stutter, and discovered that a lot of folks liked it.
What’s his doper name?
TubGuy
What’s his favourite twice-baked potato recipe?
Bake the potato by hitting it gently with a light saber.
Scoop out the cooked potato part
Mix cooked potato with cheese and broccoli
Refil potato skins
Attempt to stuff whole thing at once through mouth grill.
Wonder why everyone is laughing at you.
What’s his favourite thread?
Any that supports his Earth counterpart, George W. Bush
Does he prefer kittens or puppies?
Puppies. They’re harder to accidentally crush with your super robot hands.
I thought he only used the force when he had constipation.