Make vector-based illustrations on your phone/tablet:

Adobe Ideas is the app you'll need to download. You can easily transfer your finished artworks to Photoshop or Illustrator for further changes. It's free and running on iOS but for Android they no longer provide updates and bug-fixes. For better results, select After Stroke smoothing from Preferences. You can also use Adobe Illustrator Draw. Both are basically the same thing but Draw gives you more to work with. Ideas is recommended for anyone who's just starting out.