Brighton is an amazing own full of life and entertainment.
On sunny days you could spend the day sunbathing on Brighton Beach which is huge and stretches for over 5 miles but there aren't shops all the way along so be careful where you do decide to settle for the day. It is a pebble beach, not sand so make sure you have something on your feet.
Brighton Pier is great, there are sweets and souviner shops, game and food stalls, the Palm Court restaurant, Horatio's Bar, the arcade dome and a mini theme park. I'd set aside a good few hours to gor ound this as there is so much to do. I love the little souviner shops to get little trinkets and sweets. If you want to go to the theme park part, check out what rides you would like to go on to see if it is worth you getting a few tokens or one of the unlimited rides wristbands.
If you want to stay overnight, I would suggest finding one along the seafront or a few streets away so that you are close to everything. There are usually some good deals on GroupOn and Wowcher so check there first. I stayed at the The Istanbul Ev Guest House which is a 2 minute walk away from the pier and is a nice little hotel.
If you want to go and see a show, check out the Brighton Centre, Concorde 2, Komedia, Brighton Dome, Brighton Music Hall and so many more. If you just want to go out and enjoy live music, most of the pubs and bars in Brighton have bands or DJs playing.
The Sealife Centre is one of my favourite places to visit, it's not a huge centre but what they have there is truely amazing. It's best to book online, especially if there is a group of you going as you will save money. They have so many awesome sea creatures that you can get up close and personal to, plus, they have the Behind the Scenes tour where you get to see how sick fish are looked after, where their food is prepared, where some fish are bred, it's really interesting.
The British Airways I360 is a great attraction to go on in order to have the most amazing views of Brighton from above. You got up a tower in a round tube, it stops at the top for a little while which is the perfecct time to get lots of cool pictures, and then you come back down. If you got the train to Brighton, you can get buy-one-get-one-free tickets. If you are a bit scared of heights, I'd suggest sitting on the benches in the middle. You can still see the great view but further away from the edge. Once you get off the I360, you then get to look around their little museum about how it was built.
The Royal Pavilion and Gardens are fantastic to visit on sunny days as you can look around the Pavilion and then enjoy the sun in the vast gardens. The Pavilion is a beautiful palace that was built for King George IV as a seaside pleasure palace so it has lots of history to check out.
The famous Brighton Lanes are a must see, it is the best place to get records, vintage clothes, art, jewellery, antiques and so much more. It also has loads of great restaurants and bars to chill out in.