Aces are always 11, must be decreased to 1 if it would cause a UserPlayer or Dealer class to bust. If have several Aces, only make the first Ace a 1.
Natural or "Blackjack" when your first two cards equal 21. Auto-win unless the two initial cards of the dealer are also a 21.
Doubling Down (Doubling your bet based on your first two cards)
Splitting (Possible new sorta Usercase), if you have two cards of the same rank, you can split them into two different hands and place an equal bet for the 2nd hand. Then, play as normal (maybe just generate a new UserPlayer when it comes to that?)