How is Sodapop shown to be smart and athletic?

Very early in the book, Ponyboy introduces himself and the other Greasers to readers. When he gets to Sodapop, Ponyboy makes it clear that school is not necessarily Soda's forte. However, Ponyboy is very clear in saying that Sodapop is not an idiot. He tells readers that Sodapop understands everything.

Soda tries to understand, at least, which is more than Darry does. But then, Soda is different from anybody; he understands everything, almost.

As readers continue reading the book, we learn that Soda really understands people and relationships. While he may never score at the top of the charts on the SAT, he is incredibly intelligent when it comes to reading people. Ponyboy gives readers another example of how Soda is intelligent about a page later.

So Soda and I stay out of trouble as much as we can, and we're careful not to get caught when we can't.

Ponyboy admits that Greasers have trouble with the law every once in awhile. Then, he tells readers that Soda is careful enough (smart enough) to not get caught. This means that when Soda is going to break the law, he does so in a way that he can get away with it. That shows intelligence. Perhaps it is directed in a less than worthy direction, but it shows his intelligence.
As for athleticism, it is Darry that gets the compliments. Ponyboy is very specific that Darry is a wonderful athlete, but we are given hints that Soda is a coordinated athlete in his own right. He might not play sports on a team, but he is still a talented athlete capable of incredible feats of strength and agility.

"Yeah!" screamed Soda as he too did a flying somersault off the steps. He flipped up to walk on his hands and then did a no-hands cartwheel across the yard to beat Darry's performance.

Sodapop is Ponyboy's older brother, and he is described as being extremely handsome and funny. Even though Sodapop does not excel in the classroom (he ends up dropping out of school), there are several scenes throughout the novel that depict his intelligence. Sodapop demonstrates his intelligence through his ability to understand and relate to both Ponyboy and Darry. Sodapop acts as a mediator between his two brothers and continually resolves conflicts between Pony and Darry. Sodapop is sensitive enough to sympathize with Pony's situation but also mature enough to appreciate Darry's sacrifice. Toward the end of the novel, Sodapop attempts to repair his brothers' strained relationship by lamenting about their constant fighting and reminding them that they are siblings.
Sodapop demonstrates his athletic ability before and during the rumble against the Socs. Before the rumble, Pony describes how Sodapop flips in midair down the street and walks on his hands. Sodapop is also portrayed as a good fighter and holds his own during the rumble.