Need day out ideas for DS's 6th Birthday, quick!

[Tooawesomeforausername]

We was taking him to Disneyland ( all booked and payed for) cake at cafe Mickey, he was so looking forward to it, but my idiot Ex mucked up with he passports, so we can't go
Everyone has given him his pressies early as we wasn't going to be here, can't stand the thought of him waking up to nothing, so want to take him out for the day, but money is tight, so have a budget of about £100- £150.
Was thinking natural history museum and a trip to Hamleys, but would like to take him somewhere special for lunch or dinner, preferably with a cake, any ideas?

Or Chessington?

Only have a few days to decide

Any other ideas greatly received

Is there really no way you can sort out the passport? You could get a same day passport for your £150 and then go to Disneyland as planned?

We often go to the museums with our boys. DS1 is 6.5 and loves them. On balance I'd say that the Science Museum is better as it's more hands on and there's loads to do. Take a change of clothes though as there's a great water play bit in the basement and they can get pretty wet! The science museum also has a really nice cafe that does good children's food. Your budget will be more than enough as the natural history and science museums are free. Very annoying about passports!

The ex asked about same day passports, but they said no, as there is already one being prossessed also, they have my kids documents! the DC's are devistated (as am I!)

I'm thinking the museams are the best idea, maybe a trip to Hamleys, but would like him to have a cake with candles somewhere whilst we're out (will just be me and DS)