My first guinea pig was a turquoise heather ONPF T, which I transformed into the stegosaurus T (number 33 in the book, "tying game"). All it required was cutting off the sleeves, hem and neck, making one vertical cut all the way up center back, then making a series of 3-inch horizontal cuts 1+ inches apart on both sides of the center cut, then tie a bunch of knots and you're in business.
My pink ONPF T got a different treatment (number 19 in the book, "fermez la ruche"). I started by (again) saying "bye-bye" to sleeves, neck and hem. Then, I stitched new side seams 1" from the existing seam. That extra seam allowance became casings for drawstrings made from the few inches of T I cut from the bottom. Cute! A detail photo of the ruching is on my Flickr page.
When I lived in Belltown and had a P-Patch (community garden) plot, I bought a P-Patch T-shirt because I liked the color and graphics. What I didn't like was that it was a man's-style T. Now that it was even baggier than ever, I never wore it. But now, minus mannish sleeves and too-long hem, plus two new side seams to nip it in, I think I will wear it quite a lot (it's number 14, "classic punk"). I gave a similar treatment to a really baggy white V-neck (keeping the original) neck, but felt it was to utilitarian to bother photographing.
Next up: Let's see how many items of clothing I can make from a super-soft thick brushed cotton long-sleeve T of Js in a great color of red that surprisingly has barely faded (J bought it back when he still tended to wear his shirts and sweaters on the big side; now that he wears sleeker clothing , he's purging his closet). I'm aiming for a little skirt, legwarmers, hat and arm warmers.