Take your debit card if you have one, and get cash out of an ATM as and when you need it.

Travellers cheques.

I would change all of your pounds into dollars. I really do not think you can spend pounds in the U.S. You will have fun either way!! Good luck!

Dollars would be easier because it's the currency of the country you're visiting. IMHO

I wouldn't use your Barclays card in the US, you will get the least favorable exchange rate. If you plan on spending all your money, exchange it once you get there, take some of the cash and put it in a safe place, and with the remainder get a prepaid card.

Check the commission and handling charges Barclaycard will charge!
My card recently charged my £10.23 for a £300 purchase ON TOP of the poor exchange rate!

Previously I would have said use your c/cards for most purchases but not now!

Take LOTS of small notes (1USD is used as tips everywhere).
DO NOT take 50 or 100USD as they will be refused in most retail establishments due to fraud worries!

Check which version of Barclaycard you have.

I get a good exchange rate and low to no commission on mine, so when I am in the US I use a combination of card and cash.

That way the card can be used for bigger purchases and you get the added protection that Barclaycard offer.

If your card has a rubbish deal, just go with cash.