Take a bag - you WILL get given lots of reading material and having some way to carry it easily (and then keep it together when you get home) is worthwhile (often there will be bags available but they might be cheap plastic bags, they might only be available from certain stands and they will almost certainly be branded - pick one up if they look useful but having a rucksack to put everything in makes it a lot less annoying to carry around).
Take some food/drink (and some money for food drink while you're there) - open days are long, they can be very busy, if there are long queues for food at lunch time and you have a few snacks then you can keep your energy up without wasting time in queues.
Take maps to find your way there, find your way around the local area/between campuses and to find your way around uni buildings. Sometimes these are available sometimes they're not.
Take any booking details/timetables for things you have pre booked on. Some unis will restrict events to people who have prebooked, others will work on first come first served (usually means more queuing). Make sure if you've got it on your phone you can view it without mobile signal....not all campuses will have great signal for all networks.
Take your phone with charge and credit.
Take comfortable shoes, suncream if it looks hot, waterproofs/a brolly if it looks like it might rain.
Take your voice - noone there knows you, there are no stupid questions. Speak to anyone and everyone. Ask other people who are visiting if they'd recommend anything for the day, ask staff if it is really worth a 2 hour round trip to the halls for a tour and back again, ask students what their timetable is like, how they are finding halls, what the social life is like, ask staff what sort of support is available, whether you can study your favourite topic, how flexible options are etc etc.
Take a notepad and pen - on your way home or when you get home note down the things you liked, anything you didn't like, anything that stood out as interesting or important, anything you want to find out more about....if you go to a lot it is very easy to get things muddled so having some notes from the day will help when you look back at the end to decide if you want to apply to anywhere you've visited.