Race courses are funny places. Agree on something warm, particularly as the weather next week looks mixed.
Also, and conversely, if it’s sunny you will get sunburnt if it’s sunny. There is minimal shade. So consider some suncream.
Your feet will hurt whatever footwear you go for. There is always guaranteed to be more standing and walking than you expect. The ground should be dry and not too sinky but do watch for stilettos or kitten heals. Wedges always a popular race going choice.
Hats are optional, so entirely up to you. If you do wear one make sure it’s comfy and doesn’t slip off every time you move.
Also agreed on the properly closed bag/purse and ideally not a clutch as it’s a pain to carry around all day, and you wouldn’t want to put it down even if you can find somewhere to put it (back to my standing around point)
Take plasters and some safety pins in your bag, you never know.
Hope you have a great time.