Stamp collecting.
I suppose it's like sticker collecting but with the added perk of supporting the USPS. The other day I noticed that I was running low on Forever Stamps with the boring American Flag design, so I asked the clerk if I could buy a few after sending off a package. I wasn't aware that the specially designed ones were the same price as the generic ones, so I picked out the Elephants here.
I can't remember why, but I was in a rush to write my rent check a few days later and used up one of the Elephant Stamps, so it's no longer a perfect mint-condition sheet. I don't think my landlord will appreciate how cute it is. I've been devastated ever since.