After returning from my 4 day trip from Cinque Terre, I realised it can be done in 2 days. For anyone who likes to get in and get out, this is for you.
Here are my suggestions:
1. Pick an island as your main base. We booked our hotel at the last minute and found a hotel in La Spetzia. It was actually perfect for us because the hotel was literally down the street, about 5 minutes walking distance from the train station and breakfast was included :)
2. Once you arrive, find the Information Center. Ours was right outside when we hopped out of the train. There, you can buy the 2-day pass which is good for any time and you can hop on or off the train as much as you want. You can also use this pass for busses, hiking trails, bathroom entry, etc. This is definitely useful!
3. Select the island you want to see first. My friend and I agreed to go to Vernazza as it was one of the most picturesque and smaller compare to others that we heard of. Also, we came during the week when it rained in Cinque Terre. By this, we wanted to choose a place during our first day that we could cover in a short time and still enjoy it. Vernazza was perfect!
4. Allocate your time. Unless you wanted to swim and sun-bathe all day, you can pretty much stay in each island for at least 2 hours and then move on.
5. Enjoy! Never forget to have fun.