Wilton makes a tractor cake pan that should still be available for you to make your own cake. If you use the 40% or 50% coupon that Michaels has in the Sunday paper it makes the pan affordable.
If you don't want the cake pan (it takes up space in the cupboard) then decorate a regular cake like a farmers field (crushed graham cracker makes good dirt) and put small John Deere's on the cake as party favors for the kids.
Get green plastic table clothes from the dollar store with green cups/plates/forks.
Get some boxes and a can of yellow spray paint to make "hay bales" and then have the kids pull their wagon around them as if the kids are are the tractor. If you can get another kid to bring his wagon then you can have a tractor pull!
Other party favors might include bandanas and straw hats.
Go to DLTK.com and look kup coloring pages/ideas/games for ANY themed party for free. My oldest is now 7 and I still use their free site for great ideas. And when my 2 boys were 4 I planned 2 games for the party, one of which was a coloring page. They were more than happy with this idea because they helped to pick the coloring pages and design the 'big' game.