Ladies and gentlemen, I give you:
(a) South Shields.
Perfect for a day trip. Includes a lovely park (complete with tiny steam train), some bonkers weebls overlooking the beach (see above), the best fish and chip shop in the world (otherwise known as Colmans), and the pleasingly retro Ocean Beach pleasure park (which contains some rather good dodgems and trampolines). If you had a 1970s childhood, you'll feel right at home.
(b) Low Newton (Northumberland).
Perfect for a day trip. Includes a lovely stretch of sand and a nice pub (sadly rather rah-infested with Portias, Sols and the like, all kitted out in Gap and mini Boden). There's some good BBQ spots, and limitless potential for digging holes and running around throwing frisbees.
I'm not sure which I prefer, to be honest. I've had a brilliant time at both over the last couple of weekends.