Cuba para Cristo - Cuba for Christ
Money
Currency
Though Cuba has its own currencies - the Cuban peso and the Cuban convertible peso, a dollar equivalent - small UK pound or euro notes are the best cash to take. Travellers cheques can only be cashed in a limited amount of places, and it can cost 3-5% to cash them. Other foreign currencies can also be exchanged into Cuban dollars in certain hotels. Don't take US dollars; there's an extra tax on changing these.
You can draw money off a non-US credit card from a number of money-change kiosks around the country. This is not a bad way to access money if needs be. It's a bit like a UK cash machine, taking VISA, etc. but not Switch or Delta. You'll also have time to change money at the airport while waiting for your luggage.
Expenses
Fast food. McDonalds has not made it to Cuba yet - but if you do want Coca-Cola or ice creams from dollar shops, expect to pay what you would pay in the centre of London at a kiosk. If you do end up eating out, then food costs between $3-$20 for a meal, depending on where you are. But remember that locals will tend to think it a waste of money to eat out too much - what you are spending on a meal is what they live off for a month.
Scuba diving costs about $60 for a double-dive, including kit. Car hire is very expensive (> $100/day) and not recommended - better to ask a member of the church who has a car to drive you, and you pay fuel (if you want to go off just on your own). Alternatively, ask the pastor to fix up something.
There are various local arts and crafts you could buy - e.g. a flag ($15), humidors ($40-$100), cigars (From $25 upwards/box). Don't buy on the street, and if they are cheap be suspicious. Some you can pay for with a non-US credit card, others you will need cash.
There is very little else to spend money on in Cuba - particularly when in Christian circles so not buying alcohol, etc.