I’m a very UI-savvy person and I focus on the most intricate of details when it comes to UI, so here:
Your UI should have a theme and should look consistent throughout your game. For example, if you use rounded-corners here, then you need to use it almost everywhere else to make it look like it’s part of one game. However, do not make it monotone. You can change up the colors, positions, and layouts ever so slightly to make them have variation.
Satisfying UI Animations
This includes anything from hover effects to your shop UI opening. Make them seem specialized in their way while preserving consistency. For example, for a button that gives you rewards for the round, you can have a stash of coins spread out slightly when hovered over and put back when the hovering stops. Another thing for acquiring mass number items such as 50 coins, you can make several coins move from the button to the coin display in the corner to give the user a sense of satisfaction. Also, make sure you don’t use linear animations too often because they look quite boring. Quick, asymmetric, and irregular animations are key.
Audio can be satisfying to listen to, especially if you get a ton of coins, then you can have several coin dropping downs when the coins move to the coin counter. Audio brings your UI to life in another dimension, i.e. in sound and not visually.
If the style of your game is fun and cartoony, you need to make sure your buttons and everything doesn’t just stay in place being static. Make the major buttons shine using a gradient shine effect, or make them jiggle a little every once in a while. Also, when you open major UI frames such as opening a shop, make certain elements appear after. Don’t tween everything in from the top, it just looks like you’re sliding a piece of paper from the top. Make it look like you’re passing a 3D diorama giving your UI some depth.
Just look at Angry Birds 2’s UI online or download the game yourself to see, because oh man, the way they made it, it’s quite juicy to press buttons!
I don’t have a set app to use, but one thing’s for sure: get an image editor or art creation app that allows anything beyond basic creation, whether it be free or not. Most of my UI creation doesn’t go beyond Google Drawings (depending on your game style). For example, Islands has rectangular UI, meaning they technically don’t need high-feature app, though it’d great to have if they make changes later on.