I usually include something like, "We want your presence, not your presents.", because I like to think I'm clever lol. A lot of my daughter's friends will include something like "If you wish to give a gift, please bring a donation to x charity" or "a new, unwrapped toy to give to Toys for Tots" or something like that.
Regardless, some people may bring presents. Be ready to accept them graciously. Prepare for this in advance by subtly removing and setting aside a few toys your boy doesn't really play with... they can be returned if no one really brings anything lol.
Saturday afternoons are AWESOME for dropoff parties, but the general advice is to keep it small--most people say one child guest for each year of the child's life (though I disregard this when it comes to siblings). Also, keep it short--i usually plan 1/2 hour per year, and activities should be 1/4 hour per year max.
Make sure you plan to have them entertained every second--have things like coloring for transition times between other activities. Up to about the age of five, have a helper with you during the party to redirect children who become upset or stray, change any diapers, etc.
PLAN FOR A FAMILY NAP AFTERWARDS, even if the kid took a nap before it.
Good luck and I hope the party is even more fun than you hope!