Mushroom is indeed the crowd favorite, and for a reason.
Risotto alla milanese (with saffron and a good stock) is a classic.
I've made a lemon and shrimp risotto that I thought was quite good.
I also like risotto with peas, you can make it in the style of risi e bisi with prosciutto or other dry ham.
Also, when you run out of risotto or get tired of it, barley make a nice alternative. It has a somewhat different texture, but works well in most if not all risotto recipes. I use it sometimes for variety.
