The most important "trifle" that moves the plot is Minnie's quilt. As the men walk around the house trying to find clues, the women take a few things for Minnie to make her jail stay more tolerable. They see the quilt that she was working on, and discover something important:

MRS. HALE: ...look at the sewing! All the rest of it has been so nice and even. And look at this! It's all over the place! Why, it looks as if she didn't know what she was about!
(Glaspell, Trifles, etext.virginia.edu)

Mrs. Peters goes on to mention that she sometimes sews badly when she's tired, but both women instinctively know that the bad stitching is symbolic of deep stress or unhappiness in Minnie's life. The quilt also directly causes them to investigate Minnie's sewing box, where the dead canary is lying in a fancy box. Ironically, the men see no importance in the quilt at all, mocking the women for being interested in the quilting or knotting, while they look for "important" clues. Instead, the quilt is an explicit clue for Minnie's motivations.