Welcome to Marapets! I hope you continue to enjoy it here!
1. My personal favorite pets are Decadals, Oglues, and Willas.
2. All free games are worth doing in some capacity, but I try not to miss doing Archeology and Worm Digging. For the game Balloon Burst, you have the opportunity to win a Capsule Machine Token. One thing I don't think people realize is that you can also potentially win some low rarity retired items.
3. My biggest advice is to try a bit of everything, but don't let it overwhelm you. Often times, it's easy for new members to get frustrated and take a "why bother" approach, especially when it comes to collections, but I recommend not ignoring them. If you're torn between adding items to your collections or selling them, perhaps consider placing those items in your attic so you can think about it.
By the way, there is an Advent Calendar in December. It's still a few days away, but don't forget to log in to claim your prizes.
https://www.marapets.com/advent.php