Go to Avios shopping and buy something from AliExpress that's very cheap with free shipping. Having said that, AliExpress doesn't always track so if you don't get a notice from Avios in a day or two that you're Avios are on the way then it didn't work and you'll have to try again.

There might be some other shops on there that are useful to you as well. I've not really explored it since I normally use AA eShopping for that. iHerb is probably on there and tends to be pretty useful for a lot of different things. Avios shopping is pretty lucrative for hotels, they generally give a much higher multiplier for Marriott than AA and they have Hilton which AA doesn't, but those won't credit until well after your stay. You'll need to generate some activity quickly which shopping is the quickest and easiest if you're not travelling in my opinion.