Similar to MostlyBearded I have built up, and spent, a fair amount Avios using American Express cards. I thoroughly recommended headforpoints.com to find out ways of building up your Avios balance or getting hotel points for free stays.
I have used my Avios and 2 for 1 vouchers with my BA American Express card to fly in First Class with BA to New York, Vancouver and Washington.
I also use Topcashback.com a lot to get money back when buying car or house insurance, changing energy suppliers or booking hotel stays with Hotels.com. I've had £1,800 back over the last 3 years for things I needed to buy anyway. Not sure the return I've had off the top of my head but it is enhanced by getting an additional 10% when taking the cashback as Debehams vouchers, or 3.5% for Amazon vouchers. The risk with Topcashback.com is that your purchase doesn't track or you don't get the cashback paid for some other reason. I have only had a couple of small payouts not pay so not too mtotooo bother about.
I use Hotels.com for booking holidays as I am getting 10% (ignoring taxes on hotel stays) back on hotel bookings as for every 10 nights you book and stay via Hotels.com you get the average price paid for those 10 nights to use towards another booking. Although not a risk if you have any status with hotel reward schemes then you won't get any benefits when booking via Hotels.com and you can't collect reward points.