I haven't been to any of the HUGE ones, but I've been to some decent sized ones cons. Haven't been to any post-COVID though.
If you want to get the most out of it, be prepared to walk and stand a lot. They usually have an itinerary online or at the entrance, so you can plan out your day. Talk to staff about potential cancellations. Study a map of the area beforehand if you can.
Don't assume you can use a card. Credit card readers WILL go down or not work at all at some tables. Bring a variety of cash. Every con I've been to I've had to break bills for strangers and help tables with their registers if they need more ones and fives.
I always bring a backpack to hold my water, camera, and purchases. I bring shopping bags to hold stuff too. I drop things off in the car when I get overloaded.
Hopefully, they let you bring food. I usually have a banana and a trail bar, but I still buy at least one meal. Always overpriced!
One big thing I learned - if you plan to buy art or autographs, make sure to buy sleeves or some kind of art folder beforehand. If they even have them, they often run out or charge too much for them. They often won't have thicker stiffer sleeves, just the thin ones.
As far as cosplay... Haven't done it! Make sure your costume is compatible with the local weather. I've seen a cosplayer suffer from heat stroke, and I've seen others shivering for extended periods of time. Be prepared for wanted and unwanted attention. Staff is there to help.
