Here are my top 3:
Most unusual, for me anyway, when I was a very young kid and had recently discovered the pleasures my anus could provide I was desperate to find something but too many of my family were at home. So my usual go-to of grabbing a carrot and carving it into a fun shape before pushing it inside me was not an option. Frustrated, I hopped on my bicycle to distract my mind, but as I rode down our country road, I spotted so many possibilities near a swamp ... cattails(bulrush)!! All different lengths and sizes, still on the green side so they were firm and some even had some interesting ridges and bumps.
I picked several and over the next few days when I had some private time, I would make sure to cut off any sharp spots, wrap one in plastic wrap, smear some vaseline jelly on it and push it into my hole. Very enjoyable.
When I was even younger, I would lay naked on my back on our bathroom vanity, vaseline my anus and the tap of our sink (which was about 4" long and positioned horizontally) and gradually shimmy myself toward it so I could penetrate myself with it ... my parents still have the same vanity and taps and I realize how small I must have been to do those maneuvers lol. My anus gets a tingle every time I wash my hands there.
Last, and still on a very rare occasion today, I have a really heavy framing hammer with a thick rubber handle that has a large tapered wedge shape at the end to prevent it from slipping out of your hand - the odd time that big old handle gets a condom slid over it and finds it's way into my backdoor. The weight feels great!