I have done this two years ago, and I would do it again. Packing is difficult you have to include a lot more stuff than you would with a reg. child (diapers, burp cloths, ointment, gas drops-don't forget, and infant tylenol-another must have). I brought a white towel to the parks with us and draped it over the stroller canopy (they are usually dark colors and absorbs the sun) and while he was sleeping I hung it down so it shaded him from the sun, I also used it while I was breast feeding for shading him and me.
If you are going mid summer (we went in August) they have these great onsies that were like tanks on the top they were great. Someone else on this board suggested (I wish I knew then) getting a portable fan (operated with batteries) and clipping it to the carriage. Get a great backpack with lots of compartments, I have found that they work MUCH better than a diaper bag and you can hang it off the carriage. Bring more wipes than you think you will use for wiping sticky hands and tables for that matter. Bring an extra empty bottle and fill with room temp water, don't use cold thier tummy might cramp up. Thats all I can think of for now. They won't remember anything at that age, but they do react to what they see. I remeber when we went to Universal Islands of Adventure, the Cat in the Hat loved the little one, whoever was in the costume was awestruck at our little guy. We got loads of pictures with them together, he may not remeber but we do and I am sure he will love the pictures when he gets older.