Talk about a satisfying little project.
Take one t-shirt, never been worn because it is just a little bit annoyingly too short.
Take less than half a ball of soft Patonyle sock yarn in a matching colour.
Swatch one or two edging patterns from a book, pick one and work out how many repeats you need to go around the bottom edge of the t-shirt.
Knit the edging and hit it with a good blast of steam to make it sit nicely.
Hand-stitch it to the hem of the t-shirt. And you're done.
Aside from swatching a different edging option the day before, this was completed in one day (and I didn't spend the whole day knitting) and I wore it the next.
The pattern is Seashore edging from 'The Hamlyn Complete Knitting Course' by Eleanor Van Zandt.
I deliberately chose not to block it hard to its full lacy potential because I wanted to finish it quickly. Then I backwards-justified that decision, thinking that, this being a t-shirt, I'm going to be machine washing it (in a lingerie bag) and the most I will be willing to do is a quick blast of steam from the iron before running for the bus in the morning. I'm hoping the sock yarn will be a good choice to stand up to a fair bit of wear this way.
I hand-stitched it to the wrong side of the hem. Another option, especially with a more lacy edging, and if you wanted a more fancy look (maybe even with contrasting yarn) would be to sew it on the right side, partially overlapping the t-shirt so that the fabric shows through the lace.