Go out! Seriously, it's so much more fun, even if you just grab a quick dinner and coffee with a friend. I'm naturally prone towards severe introversion when I'm busy and stressed (my way to rewind), but if I have more than a day off after a busy period I quickly sink into isolation. I had to force myself to have dinner with a friend tonight, got coffee afterwards, had a great conversation and even met up with a PD officer I needed to talk to anyway.
Moral of the story: take your friend up on it the next time he offers. 😛