Brighton Pier

If it’s a sunny day, after you’ve walked along the Palace Pier, get sandwiches and picnic on the beach. Make them dip their toes in the waves. Walk along the seafront towards the i360 and Buy ice creams from your favourite gelateria. Feed the seagulls/ throw stones at the seagulls (delete as preferred). That should take up 3 -4 hours.

…past - you have 3-7 food stands (including burritos if my memory serves correct), a couple drink stands/bars, and several open tents for shelter. There doesn’t seem to be much entertainment other than the events itself, so if you arrive early it may be worth checking out Brighton Pier (it has an indoor section for arcade games- a life saver if it’s raining), or The Lanes (a region of shops, cafes, and bars local to the seafront).

…Beyond Retro... probably loads more. They usually have Instagram accounts so maybe have a look and see if they look any good. And for other things to do, depends what kind of thing you like obviously but looks up the following and see if it's up your street: Royal Pavillion. Palace Pier. The Lanes (another shopping area). Boat trip from the Marina. Rent a kayak/paddle board if the sea is calm. Get the bus to Devils Dyke or Cuckmere Haven. Live shows ranging from local bands playing for free in a pub to the London Philharmonic at Brighton Dome, there are tons of smaller venues.