Sugar free/diabetic cooking

I'm looking for some tried and true cakes/biscuits/sweets recipes that are sugar free (so use a subsitute like splenda). My friend is diabetic but also a (self confessed) crap cook so I'm going to give her cooking lessons and help her to bake a few sweet things too. I'm just not sure whether you can directly substitute the sugar for splenda, I plan on having a play around but wouldn't mind being set in the right direction ... anyways, any suggestions greatly appreciated! TIA!