I was hoping someone would ask... Dinosaur Adventure!
It was a genuine full day out for DD(4) and DS(2) - really large adventure playground, then a dinosaur trail through the woods, through to lovely animal section/petting zoo. Was really quiet when we went (term-time), staff were friendly, great animal demos throughout the day, deer safari (extra charge but worth it for ride on large army trucks).
The only thing that lets it down is the cafe, but we took a picnic and there are loads of tables, plus a covered area if the weather does it stuff.
And you can do it on Tesco tokens, so cost is minimal.
Oh god, the tank museum... I let DP take us there on the last day. I thought I was hiding my excruciating boredom quite well until another woman walked in with her husband and family, saw my face, let out a scream of laughter and said 'That face says it all!'.
Cromer was a big hit with us - great beach with surf school for wannabe dude-watching, fab pier you can go crabbing from and visit lovely RNLI lifeboat house and museum. Couple of nice coffee shops, too and a lovely gallery/nice things shop called the Garden House near the front. Not forgetting the hop-on, hop-off land train that my DCs loved.