One great way to brush up on skills is to do some flash cards for addition and subtraction!

A website my students have enjoyed in the past (to learn math) is called Prodigy Math.  It's a game like Pokemon that mixes in mathematics.  You can set the level intensity as you'd like!

For math fact practice, (it does cost $3 I believe) you can use Operation Math on tablets and phones.  It has a bunch of different settings and can be a blast for practicing.