This is the most guaranteed way ever to get points. Each potion you make gives you five points: however, you can only make two a day, which you in turn document on the House Points chapter. Here's a example.

Me: I earned five points for Slytherin.

(Five hours later)

Me: I earned five points for Slytherin.

The catch? There is none! Thank Pottermore for embellishing my love for a good potion. Brew them, bake them, love them.