You can add all sorts of different things to playdough to make it interesting. You can use either home made or shop bought, but home made is probably better, as shop bought stuff has its own smell that would distract from the smell you were creating!!!
Eg's: For Chinese new year we added 'chinese 5 spice' to the playdough. It was an unusual smell, and made the playdough feel different too. After Diwali, we had some chopped fresh coriander left over from another activity, so added it to the playdough! You can add all kinds of lovely smells, such as vanilla, cinamon, peppermint. And what about adding glitter for sparkly playdough.
If you make your own play dough, make it a plain dough colour, and allow the children to add colour themselves. They love to watch it change colour!