Diet Rite is your best bet for soda; it's sweetended with Splenda instead of aspartame & is caffeine free. There's nothing wrong with drinking things other than water, but remember these don't count towards the 64oz you need to drink daily of water. You can also buy unsweetened Kool-Aid & sweeten it with Splenda, Splenda measures cup for cup the same as sugar. The bottled pre-mixed Crystal Light is sweetened with Splenda too, but the mix is aspartame sweetened. As everyone already said water is best, but it's good to have some flavorful options too.
God Bless,
Amy-Lynne