Ugh! I bought a packet of sweetened soya milk by mistake instead of unsweetened.

The unsweetened variety is lovely on cereal in the morning but it's not nice sweetened!  
I've always felt that if people want to make something sweeter then they can add sugar (honey, syrup, dried fruit, apple juice, whatever) themselves and don't need it added by the manufacturer.
I've used the rogue packet of sweetened soy milk to make a rather nice rice pudding in the slow cooker and here's my recipe:

SOYA RICE PUDDING
900ml sweetened soya milk
6 level tablespoons pudding rice
1 tsp of butter (or vegetarian margarine)
1/2 tsp cinnamon

Cook in slow cooker for a few hours - it's very nice.  Or you could use part soya milk and part creamy milk.