Get a BA credit card with a sign up bonus

Buy them online

Make a small purchase through shopping.ba.com and hope the miles post in time (if UK based)

Credit a Hilton stay, do a cheapo 1-day Avis rental etc

Transfer some Amex Membership Rewards points

Transfer some hotel points from Hyatt, Hilton, Marriott, Priority Club - assuming your surnames match it should go through

I think (think, note) that booking a reward flight and then immediately cancelling it counts as activity as well