I'm just loving Slonimsky's Thesaurus of Scales and Melodic Patterns, which I use as an alternative to boring old major and minor scales and arpeggios. Like "bitonal arpeggios" (alternate C major and C# minor, for example.)
But best of all is I just discovered some great miscellaneous stuff in the back. There are great dodecaphonic patterns and polyrhythmic scales, like 3:2, 4:3, 5:3, and 5:4.
The 3:2 one would be great if you want to work on 3 against 2. Just play a C scale in triplets in the right hand and duplets in the left hand. If you do 2 (or 4) octaves, you come out even at the top. Then switch.
If that's too dull, try doing C in duplets in the left hand and E major in triplets in the right. Cool! (Make sure the neighbors can't hear you though.)
Topic: Practicing 3 against 2