A dollar store poncho while likely not needed for rain, can actually come in handy if you want to be on the rail in the wet zone for WOC in the cooler evening. It can also come in handy for GRR if you have people that don't want to be soaked, or are riding GRR before a dining reservation or something else where you will will sitting inside. Not fun if you get off that ride looking like you just fell into a swimming pool with your clothes on, and then go inside somewhere.
Expect cooler in the mornings and evenings and hot in the afternoon. Last October, we wore shorts and short sleeved shirts, and I don't recall ever getting to the point in the morning or evening that I wanted anything else. I remember thinking it was really nice in the morning and evenings and a little hotter than I would like in the afternoon (cooler than most of our October WDW trips though). I did wear long sleeves to OBB, but that was costume related. I would think September would be similar to the first part of October. I have had times in June where I have wanted a sweatshirt at night. It may depend somewhat on where you are from and what you perceive as cold. You may not need a sweatshirt or jacket at all--- but if you are in a tank top without sleeves at all, you might. I balance whether I want to carry around a sweatshirt all day when I may only need it once it starts getting dark, with my cold tolerance. Last October, I decided I would just use the cold as an excuse to buy a sweatshirt (which I have done in June in the past), but it never got to the point that I felt like I wanted long sleeves.
Sunscreen is a must and we always use body glide on our feet to avoid any blisters. I'm blister prone and I haven't had a blister at DLR or WDW since we started using it years ago.
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