There are so many math apps out there and not all of them are awesome. And some are, but you can only do so much before you have to pay for full access.
5 Dice is a great FREE app that helps students practice all levels of Order of Operation.
Then they have to create an expression, using all the numbers, to hit the target number.
Once students hit the target, they are shown the expression they made and how it is simplified.
My students think it is fun and it is a great way to practice order of operations.