I've been taking weekly photos of my tummy, side on, and front on while holding whichever fruit The Bump's foetus size chart says is an approximate size of our foetus. Looking ahead in the chart, I've been wondering what to do when i get to the weeks that compare the foetus' size to different varieties of melon. I don't like melon and don't want to waste one by buying it just for a photo. Hubby doesn't mind cantaloupe (rockmelon) so i can do that one as per the list.
Some ideas i've come up with are pineapple (up to 12 inches or 30 cm) for weeks 22-24, replacing a papaya. Cabbage seems to be about the same size too.
I can't find anything that is obviously honeydew or watermelon size. At the moment i can only think of finding some rather large pumpkins, and possibly a very long chinese cabbage.
ETA i've just found another few very good sources of fruit comparisons! http://www.babycenter.com/slideshow-baby-size and http://www.parents.com/pregnancy/week-by-week/how-big-is-your-baby-this-week.