Soda is easy, depending on what you want to make. The easiest route is to buy the soda syrup, either a small bottle of extract, like this:
Root Beer Soda :: Midwest Supplies Homebrewing and Winemaking Supplies
and follow the directions. If you like Sprecher sodas, they are available in the big jugs:
What I've been doing, though is making ginger ale. I like it best, and it's easy. I only make 2 liters at a time, but you could make more if you wanted to keg it and keep it on tap. I bought a carbonator cap, and it fits a 2 liter bottle well. You just mix up the ingredients, and then put on the carbonator cap and then hit it with 40 psi or so until it's carbonated.
A couple of things I learned- rootbeer flavor does NOT come out of your beerline, so make sure it's a dedicated beer line. Secondly, soda should be served at a much higher pressure than beer, like 35 psi or so. To not get a ton of foam, you need a super long beer line. I used 30 feet, and found it cheap from mcmaster carr (.10/foot or so). That's one reason I like the 2 liter bottles- no keg/beer line, etc. But I'm the only one who drinks soda, besides my son who made root beer and simply bottled it.
I posted the ginger ale recipe the other day- if you want, I can dig it up. I'm thinking that it would be nice to do a lemon/lime soda for spring but I haven't tried it yet.