London for the day

[MabelthePenguin]

I'm just booking train tickets to go to London for the day on the 27th August I've not been since I was little and I've wanted to take my boys for a long time, ds1 is 9 and ds2 is 3.
Has anyone got any tips or any advice on places we must visit, were coming from Manchester to Euston.

shannie enjoy Matilda, it is fab. Saw it at Stratford before it transferred.

So excited. Not got great seats, it's so expensive isn't it but decided to just go for it.

Get there early a borrow a booster seat as its quite a flat theatre

Great tip ( another one), they got binoculars haven't they. Thanks

An update from the train home:
Thanks so much for all advice, esp Flik for typing out all the walks etc.
We did Skygarden, walked round tower, saw monument over bridge, south bank and Tate modern.
Sat - Thames clipper, cable car and dlr, then Matilda.
sun - palace and the walk that was suggested. Lunch and a general chill in St Martins. Quick 5 mins in national portrait gallery then Euston train home.
We are exhausted. Heat was crazy but had a wonderful time.