How do you ripen up a pineapple?

Pineapples are non-climacteric fruit. This means they do little to no ripening after being picked. They will get soft as all fruit will over time, but the sugar levels at picking will not change like some other fruits do.

So, Pick a good pineapple in the market

It's already been said, but put it in a large brown paper bag and tie the top very well. Many fruits (avocados, oranges, tomatoes) produce ethylene gas which, when trapped around the fruit, will speed up the process.

Edit: Actually, pineapples can ripen after being picked. The trick is to try to peel off a bit off the tough outer layer (the prickly parts). Under-ripe pineapples still have a lot of sugar in them; by softening the fruit (which helps break down cellulose), you can taste them more easily.

Mark is correct. Pineapples do not ripen after picking.

Put it in a paper bag and close the top. The ripening process of fruit is caused by the fruit giving off a gas that makes it ripen. Putting it in a paper bag traps the gas inside and ripens fruit quicker.

let it sit on the counter for a few days....or a paper bag