What substances are good for spherification with algin?
Basically anything that's liquid. Fruit is very common, but we've also done herbs (mint and sage specifically). I've also had vegetable spheres like pureed peas and beets as separate spheres. At restaurants, I've had beer sphere's served with pretzel (at Cyrus) and I've seen cointreau spheres used in a drink. 

They pop like caviar, and that's what releases the flavor, so they either need to be quite large or have a strong flavor. Otherwise the taste is lost and you're left with just the (fun) "texture". It also often helps to use some food coloring if your liquid isn't a strong color. With small spheres, light colors will disappear and you're left with relatively transparent spheres. I'd also recomend trying reverse spherification (i.e. mix calcium chloride with your liquid and then drop in to an alginate bath), as this doesn't use your flavored liquid to make the shell, leaving you with more "pop". 

So look for a strong flavor and a nice pairing. Make it easy to get the spheres in to your mouth. It's easier if the spheres are on another piece of food, as cutlery will often break the spheres. The best thing about spheres is presentation and the caviar pop, so use that experience and look to your advantage and feature it in whatever you're serving. Here's my Sushi Dessert revolving around spherification.