Southend wouldn't be my first recommendation for a summer's day out. It's got hardly any real beach (but a hell of a lot of mud, as the tide goes out for over half a mile) and is full of lots of noisy amusement arcades and takeaway burger places. Many of the pubs seem to be designed to attract the types of people who drink to get drunk, rather than being pleasant social venues. Think of the worst that Blackpool has to offer and then relocate it close to London!
HOWEVER, if you're thinking of visiting Southend outside of the main tourist season, and you don't mind hunting out some of the nicer places there, you might well enjoy your visit.
Southend has two fantastic theatres, namely the Cliffs Pavilion and the Palace Theatre (which is actually in Westcliff-on-Sea but Westcliff is effectively a suburb of Southend anyway). They share a common website:
https://southendtheatres.org.uk/Online/default.asp
I've driven the 65 miles from my home in Suffolk several times to see touring productions at the Cliffs Pavilion and I've never been disappointed. My only visit to the Palace Theatre was highly enjoyable too.
The central shopping area in Southend is rather boring, with all the usual big stores (or, at least, those that have managed to survive Covid), but if you explore out along the London Road you'll find some interesting little independent shops there. The shopping areas in Westcliff and (particularly) Leigh-on-Sea, which is also really a suburb of Southend, are also worth checking out.
Southend isn't full of historic places but the pier has an excellent museum (although you need to check when it's open) and the pier itself is worth a visit during the quieter part of the year, even if just to enjoy the train ride to the far end of it. There are also a couple of historic houses (Southchurch Hall and Prittlewell Priory) within the borough.
To find some really good restaurants (including some excellent seafood ones), head towards Westcliff and Leigh. There are also some really nice pubs out that way, which are totally different to some of the rougher establishments nearer to the centre of Southend. (TripAdvisor can be your friend here).