T5 is the best of the bunch. Full range of eateries, some bars, shopping, wifi etc etc. The BA lounge is OK, busy but good enough. Not sure that counts as being entertained but compared to some places... 9 hours though.
You could head into London. Jump the Heathrow Express and you are into Paddington Station in 15 minutes. Trains go every 15 minutes as well so no hanging around. At Paddington you can get straight onto a Tube or jump a cab to take you somewhere a bit more scenic. Nowhere takes very long from here. Tube is probably the best bet.
So you have time for breakfast in the city. Wander about, Lunch then back on the HEX and into T5. It is real easy. Security is a bit of a pain but what's new.