Shoulders
As intelligent as Donatello was, it didn't take much for his brain to shut down. April was the main reason for that happening. For example, the first time that he saw April's shoulders.
It was summer and she was wearing a camisole top, only a thin strap on each shoulder holding the top up. He tried not to think about what bra she was wearing that wouldn't need straps or maybe she wasn't wearing a bra at all.
It was seeing unseen skin and imaging what was underneath her top that had caused his mind to blank.
Her time training with them and Master Splinter could definitely be seen when she wore clothes that showed off more of her upper body. Her shoulders and biceps were very toned, something that Donnie complimented her on, enjoying the way her flush didn't only cover her face, but her chest and shoulders as well.
He offered to train more with her, enjoying watching her move and put her new muscles to work. He even offered to loosen up the muscles in her shoulders after they sparred.
"You've got such good hands," April had moaned in the middle of one of the massages.
That had led to him finding out that April really liked his hands.
Shoulder massages became a regular occurrence for them after that.
It also led to the first time he touched her breasts, just through the fascination with her newly toned shoulders.