There are a few examples in the first part of this novel of how Finny takes advantage of life. The first example is how he enjoys himself so much at the beach. Gene describes how Finny has such a great time at the beach as seen in the opening quote. He says he runs all over the place and jumps into waves, laughing at the seagulls, when most others would be lying on the beach being lazy.
Another example of Finny living a full life is that he makes up his own game just because he doesn't want to waste his time playing badminton. The strange thing is that this game turns out to be a popular game in the novel which is still played 15 years later as the author says. Finny invents this game just on a spur of the moment, making up the rules as he goes along, you can see that he surely doesn't want to waste any of his ability. Which points you to the last example.
The last example is concerning his incredible athletic ability. Finny and Gene are in the pool one day, and they read the plaque that holds the swimming record times. Finny looks at one and decides that he can beat that time record. He tries, and he beats the record. Gene wants him to do it on front of an official judge. Finny Refuses and says "No, I just wanted to see if I could do it. Now I know"(35). This tells you that he lives life to it's fullest, since he is just doing this to see if he can. Most people wouldn't even think about trying to see if they could break the record, they would be too lazy to go find a stop watch anyway.
He runs around and has fun all day at the beach when everyone else is just lying around being lazy, he doesn't want to waste his time playing boring games like...