Southend is a bit of a dump. It's got the same shops that you'll find anywhere else, a seafront that has more mud than sand, the type of pubs where you need to keep an for low-flying bottles and a general air of seediness. If you do venture there, however, I recommend visiting the delightful little museum that is on (or, more accurately, under) the pier.
Bath is a long way for a day trip, but worth the effort:
http://www.visitbath.co.uk/
Rochester is a very pleasant town. (It was a city until the council forgot to observe the terms of its charter!). It's mainly got independent shops (because the big chains are all down the road in Chatham and Gillingham). There's also quite a bit of historical interest:
http://www.medway.gov.uk/tourism/44632.htm
In particular, the
Historic Dockyard (Chatham) is just down the road:
http://www.chdt.org.uk/
Among Kent's other offerings,
Leeds Castle must be near the top of the list:
http://www.leeds-castle.com
National Express operate a daily service (No. 021) to Leeds Castle at 0900 from Victoria Coach Station. The combined return fare and entrance ticket costs �22. The return service leaves the castle at 1505. (Alternatively, take a train to Bearsted station. A coach shuttle operates from there to the castle).
The
Isle of Wight is a pleasant place for a day out. Take the ferry across from Portsmouth and then explore using the little rail line or the buses:
http://www.iwight.com/just_visiting/
Chris