Heavypaint is a fantastic abstract drawing tool that's fun to do while you're killing time. Here's a great example of how easy it is to get into: (https://www.youtube.com/watch?v=yL3ho4Vc7Ng)

On my iPad, I have briefly used Amaziograph and Kaleidacam. Other apps are Hyperspektiv, Glitche etc.

You can also buy the iPhone version of Procreate which is a super sweet painting app.

Besides that, I use Garageband and Beathawk on my iPad to compose music.