Google has updated Play Games with a new UI and has added mini games to the application, making it possible to play "built-in Google games" that are listed within the app, alongside being able to launch all other games installed from the Play Store within the app as well. So far there are only three different mini games which you can play, and these include Solitaire, Cricket, and PAC-MAN. Google doesn't mention anything about the new UI or the mini games in the Play Games app changelog, and there's currently no information on whether or not Google has plans to add more mini games into the app at some point in the future or keep it to just these three.
If you're someone who just has a few minutes to kill and want to play something super quick without launching into an actual game, these built-in Google games will be a boon. As for the new user interface, everything is actually quite noticeable. As soon as you open up the app for the first time following the update which brings you to version 5.3, all of your games are now shown in a carousel-style list which you can scroll through from left to right and right to left, each game showing up as a large square-shaped card.
A brief amount of information is listed below the game card, such as how many achievements there are and how many you have obtained out of the total. There is also a play button right next to the listed achievements, and just above that you can see the last time you played each game. You can also see your entire library of games in a list down below the main carousel list of game cards by tapping on a tiny arrow icon that brings up the list on screen, which you can scroll through vertically. The profile page has also changed visually, with larger text for your name, though the backdrop graphics that used to sit behind your avatar as this is now white. The update should already be live on the Play Store though it may still take some time to roll out to users.