A splash of cranberry juice also adds a nice burst of flavour.
You could also add the fresh cranberries to the wine mixture to cook at the same time, rather than adding them just before serving.
There might not look like a lot of spices in the slow cooker, but trust us on this. They all have such strong flavours, so you don't need to go overboard.
Make sure you strain all the spices from the wine mixture, as nobody was soggy cloves in their drink.
To keep the mulled wine warm, turn your slow cooker to the warming setting and it will keep warm for the evening.
You could add some halved strawberries for a bit of a summer twist to this.