We visit Brighton about every 6 weeks. It's a bit like an English New Orleans.
The city is fantastic, and as everyone else mentioned, there's plenty to do. The Lanes has fabulous shopping and restaurants, as well as the seaside entertainment.
If you want your trip focused around the sea, you may want to try not going on a cold, rainy day. At the seaside the rain tends to turn horizontal as it gets very windy all of a sudden.
If you go and good weather is predicted but it turns out nasty, you'll still have plenty of activities to keep you occupied.
Laura