I have no clue about the up front cost for kegging, and what equipment you have that would suffice. I've heard it's the fastest.
Using yeast takes about two days beforehand, and it's best if you've done it before. But no biggie. With soda, just make sure it tastes good before you carb it, and it will taste great after. I personally dig the yeast flavor, but to each his own.
This forum is filled with recipes and links to recipes, good ideas, experience, etc. Reading past discussions is very enlightening, soda wise.
The other, more normal option, is the dry ice method. Great for parties, same day application, and you get slushies later that day, if you don't run out beforehand. You can dry ice most anything, i think. I've got a dry ice lemonade recipe that I always get asked for whenever I make it.