Several of these function like dancing in Late Night. Your Sim will appear better at it and perhaps have more fun and get moodlets. There's nothing too deep here so when I get to them, they'll ultimately all share the same page. We didn't get a traditional skill with this EP. These skills' use is largely to have fun, but you can get better to defeat other Sims more often and win more Festival Tickets to exchange for prizes. One that is not really a hidden skill is your Sim's performance in the pie/hot dog eating contests. As you compete in more of them over time, the Sim gets better and will be more likely to win (seen in game code). You'll get 25 for winning, 5 for losing. One that Sims do not get better at (it seems) is apple bobbing. You'll win tickets for a victory but can't improve.
What I know about actual hidden skills:
Soccer is an easy way to score tickets - 25 for the winner, 5 for the loser. 10 each for a draw.
Skating (ice/rollerskates) gets you 5 tickets just for doing it. Either/or uses the Skating skill.
Water Balloon Fights/Snowball fights earn 25 for the winner, 5 for the loser and share the same hidden skill.
Diving - not involved in Festivals but it seems Sims will perform better dives.
Snowboarding - haven't yet done it but I see it's 5 tickets for participating if you do this at a Festival.
You can see pics of several of these in action (no snowboarding) on the
Seasons Summer Guide.
When I make the page for all these, I'll determine the max skill levels and any traits that affect performance. Given you see all the Ticket rewards there, they seem to have been implemented with that purpose in mind. Sorry, no Soccer championship lifetime wish for us.