A standard deck of 52 cards should belong in all purses, backpacks, bags, and emergency kits. When you're stuck with nothing to do or waiting for something to happen, card games are perfect for passing time smoothly and merrily. They also make great icebreakers, and by all means, they're great for playing just for fun without any time-passing intentions!
Whenever Silkworm and I play cards, we always resort to a specific three games. For each, all you need is a standard deck of cards and at least two players (three is best for I Doubt It, but there is a two player version). Carry a deck of cards with you at all times as I suggested and all you'll need to find is someone to play with!
Easy peasy and entirely based on luck, so if you're having a good day, make a bet
*Since it's based on luck and there's no opportunity for strategy, you could potentially play a fair game against anything--an imaginary friend, a stuffed animal, another human who doesn't feel like sitting down for the game or lives too far away to be there... Just deal them their allotment of cards and play for them!
Fast paced and takes split second judgements
3. I Doubt It (also known as BS)
Players: 3+, potentially 2
Takes concentration and memorization skills to play with strategy
I have to make a note that Spit is by far my favorite. I would play it every morning and every night if someone volunteered to be my Player 2. I Doubt It comes in second place, and War in third, but I'm such a card game geek that I would jump at the chance to play any game. At least Silkworm is nice enough to play with me sometimes!