for ds1's 4th birthday last year, i invited his 2 best friends and their mums over one friday for fish n chips. that was it! he loved it and the kids all had a ball.
for ds2's 3rd birthday i invited a handful of his friends to meet at a small park at 3pm for icy poles and cupcakes. that's it! don't feel guilty, you don't always have to do something massive!