How exciting! We have just been through party season here! 3 yo parties have generally been from 10.30-12 here, to get the kids before nap time I guess.
One of the best parties DD went to was a gingerbread man theme - starting with homemade invites. The kids had a quick morning tea - not too much food which was just perfect. They then decorated gingerbread men/ladies with lurid pink/green icing and all sorts of sprinkles/lollies etc. Kids had a play then cake then back outside to run around. It was so simple but the only party the kids were not too overwhelmed.
Other's had pass the parcel, musical statues, musical chairs - all were interesting but the kids got the hang of it more than I thought they would. A pinata was also a great success - with help of course.
Cakes - the number 3 with smartie border was cute, round cake with sprinkle stencilled on using cookie cutters, Disney decal on round sponge (avail at Kmart/Spotlight/Toys R Us) and a square cake with Nick Jnr characters placed on it. Also Pink cake with marshmallows on or freckles always looks beautiful!
What activities did you want to do? Maybe pick something to tie in with that. A teddy bears picnic is always fun too.
Enjoy all the fun!