I'm spending 3 days at Heathrow (Fri/Sat/Sun) staying at a hotel there.
Now, on the Saturday, I'm planning on visiting the IWM and perhaps a few other tourist bits, weather depending. I'd also like to get to City on the Friday, which will involve the DLR as I understand it.
For these three days (I'll be using the bus to get round LHR if required! (I.e. 09s in use)) what do I need? An Oyster card? Pre-paid travel card? Something else? Nothing?
Help! You Southerners can't just make it simple, can you?!

Thanks in advance

Chris