Some fun things that we have done with our boys:
1. Cheerios - Put them in the toiet and let them "sink" them.
2. Stickers - a nice reward on a chart
3. Trees - Our boys LOVED going outside to "water" the trees.
4. Books - Get a good book about going potty and read to them while they sit on the potty.
5. Praise - Praise them a lot and often...even when they make a mistake, still encourage them.
Hope this helps!