I've done it for several years with great results.  Basically any strongly colored plant matter will work.  Even something as simple as grinding frozen spinach and collecting an aqueous extract works ok.  Flower petals work great, especially roses (IIRC, red ones give three or four distinct color changes).  One thing I introduced last year was to have them start with solid plant material and do their own extraction in the solvent of their choice.  It generated really interesting discussion when groups using the same plant material got an indicator with different properties because some used water and others used alcohol and others used butanol/hexane.  Great opportunity to tie in polarity and solubility.  
 
The other popular choices are beet juice and grape juice.

Also I have used blueberries (the juice molds really fast!) and red onions.  They work pretty much the same as red cabbage, but not as vibrant colors.  If memory serves, the blueberries gave pretty much just pink or blue.