I am with Sled on this one
My work doesnt require a sick certificate if its only 1 day.
If however its 3 or more days then i definately need a doctors certificate
So i am guessing its the same for you as well craig.
And as for excuses:
1. a massive headache
2. Diarhea (not sure of the spelling lol)
3. bad cough, runny nose and general soreness (possibly from something that you ate over the weekend, blame some restaurent for that)