If you're talking about DXM-containing cough syrup (ie the kind you can buy without a prescription), MAKE SURE you do some research on what DXM does, it is not a chemical to take lightly. Your best resource will be this link:
http://www.erowid.org/chemicals/dxm/faq/ (this is basically required reading if you want to use DXM with any degree of safety). Also, if it is DXM-containing cough syrup, DON'T DILUTE IT. It will only increase the volume, nothing can cover up its taste.
If you are talking about codeine cough syrup (w/ promethazine or guafenisen), you can mx it with any thing you want, but be sure you measure each persons dose individually, and don't just pour into a punchbowl w/ sprite and drink cups of it like jungle juice, thats a sure fire wy to cause problems. Also, unless you are opiate-naive, the cdeine is unlikely to induce much of a buzz, and the high dose of promethazine will probably make you quite sleepy (its added to reduce abuse potential-by the time you've consumed enough codiene to get high, you'll also have consumed a strong, if not dangerous, amount of promethazine)
As far as mixers...sprite? Its not really something you drink for the taste, and depending on what formulation you get, it may taste quite bad (esp the guafenesin/codeine blends). Thats really up to you.
And don't mix either DXM or codeine w/ alcohol, esp. if its yr. first time.