You could also try some of the slower arpeggiated chord songs, for example:

Everybody Hurts by REM - D, G, Em and A from memory
Wonderful tonight by Clapton - G, D, C, Em and a bar of G7

They sound more intricat than they ae, and give you the chance to move between chords with a little room to breathe in between. You can vary patterns and be musical in your own way as well. There are loads of these, so once you get the idea of playing a chord one string at a time down, you can move up the ladder with it.

Green Day songs are also very easy in general, and apart form the altered tunings Nirvana are also a good et - 'About a Girl' is standard tuning and very easy - mostly just Em and G. All these suggestions can be played entiredly with open chords...