Also worth a mention here is solo Cribbage. Especially if you already know the regular game.
For the 4x4 grid game, I like to shuffle up the deck and then play three games in a row (17 cards x 3 = 51). I had a good session the other week where I came close to winning all three games (I won the first two, and got within about 10 points of victory in the final game).