For food you need small things their little hands can hold and shove in their mouths, for fun maybe the parachute game but with a sheet or tarpauline or the likes, perhaps ask for their bikes and you can have an obstacle course or they can just ride around,  music for some dancing, mini olympics where every child is a winner etc. I am sure you can figure it out with enough ideas.

You might find that people are having trouble thinking of the appropriate gift for your son and are pleased they dont have to provide one. IF they feel guilty ask them to buy a small gift for a little boy or girl and give it to the ronald mcdonald house or something similar nearest them to brighten a childs day.

 At two, most toddlers are pretty easily impressed. You don't need to do much to make a big impression.  The less structure, the easier it will be for you, without taking away from your son's experience.

Don't have hard and fast activities. Have a few options on hand on the day and be prepared to include them or exclude them as fits in with the mood of the group. One game might be really popular and go much longer than expected. Others may be hard to manage depending on the age and stage of the kids there.  

Keep it really short. An hour and a half is heaps for this age group. 

For snacks, nutella on bread and cream cheese spread on bread rolled into scrolls is a hit with little ones. Pieces of fruit, sultanas and dried appricots and strawberries are like lollies if you are two. Arrowroot biscuits decorated with ready made icing and a few smarties and sprinkles is popular food at "day care" parties.

If you want to avoid gifts, one option could be to ask your guests to bring a plate of food to share and possibly even suggest this could be in place of a gift.  The downside is you won't be able to control the health aspect of the food in this case.

I hope that helps a little bit.