As of February 21st 2008, the following currencies can be exchanged into Cuban Convertible Pesos (CUC) at Havana's international airport: US dollars, Canadian dollars, Euros, British Pounds, Swiss Francs and Mexican Pesos. The bureau de change is close to the exit and there are two locations on the arrivals hall. There is an addition one on the departures floor upstairs. 

The first thing to acknowledge regarding currency issues in Cuba is that all references to these in guide books published before November 8, 2004 are flawed. Prior to that date, the US dollar was readily accepted everywhere and was Cuba's second offical currency. However, since Nov 2004, US dollars are no longer circulated in Cuba. US dollars are subject to a tax (+10%) for changing them. Credit cards drawn on US banks (including affininty cards for non- US organizations that are administered by US banks) are not accepted. 

More information is available on the following page:

Cuban Money Exchange

Cuba has two currencies: the Cuban peso (CUP)  but tourists will be given Cuban Convertible Pesos (CUC) when they exchange their pounds, euros, Canadian dollars, yen or whatever on arrival. This is normally shown with a $ dollar sign, e.g. on price tickets. 1 CUC is worth 24 Cuban pesos. Where prices are quoted in local shops, the price is the same whether you’re a tourist or a local.

Money should only be changed at official exchange bureaux or banks to avoid scams confusing the two currencies. Do not exchange currency on the streets, even if you are offered a seemingly better deal.

The CUC is only exchangeable within Cuba. Its value is pegged to the dollar - 1CUC =  $1.00 since March 14th 2011. However, as stated above, you will pay a 10% tax upon changing US dollars into CUCs. This tax does not apply to other currencies, so if you are travelling from the US it might make more sense to change your dollars into another currency before travelling.

When changing CUCs back on departure note that once you have cleared security only $ US, $Canadian, GBP Sterling and euros will be available.

NB the Cuban authorities don't accept Scottish sterling notes.

There is just a 3% fee for using a credit card .  A full explanation of this is available here:

Cuba: Currency and Credit Cards

There are not obvious bank machines in Varadero and the banks themselves are not that convenient to some hotels to find. Advice: Carry as much Canadian or European dollars in cash as you are comfortable with for changing over to CUC and take the opportunity to use a bankcard to obtain more money if it is convenient.