I mostly play my 3DS on my bed. That's why I tend to play my 3DS a lot more than my WiiU. I don't have to leave my room to play my 3DS, but for my WiiU, I have to go all the way downstairs to my basement and turn on my TV, etc. Too much effort. I'm much more comfortable in my room playing my 3DS.
Sitting on my couch, with the 3DS lying on the coffee table but with the bottom part hanging off of it. This is because I have a New 3DS XL so the cartridge thing is on the bottom and the cartridge might accidentally pop out while playing if it's touching something.
I play mainly on the sofa in the living room and on the bus. I occasionally play at work (if I'm not doing any work).
I've played a couple of times when I was walking home from the bus stop.
I played once in the supermarket when I was witing for my dad to come