The obvious thing to swap first David thought, would be his penis with the girl's vagina. Going on the girl's appearance, her permed, shoulder-length brunette hair and glasses, her 'shy' demeanor, he assumed (correctly) that she was a virgin. She was pretty, but the feminine glasses perched on her face partly hid those looks, including her stunning green eyes. The girl was wearing a pink t-shirt, a white cardigan and a mostly white, floral-print long skirt, along with a pair of ballet flats. She was a delicate thing, no doubt.
David continued giving it some careful thought, then decided to see just how pretty the girl was without her glasses. He focused... and his vision went blurry all of a sudden, only for a pair of dark-rimmed, men's glasses to appear on his face. He took them off and studied them, wondering what had transpired. Then it hit him, this must have been a reality change. He had swapped his eyes for the girl's, and he now had piercing, emerald-green eyes, but he didn't have her feminine glasses, instead he had glasses worn by a guy.
Walking closer to the girl as she kept reading her book, and when she looked up for a moment, David got a good look at her. She was very cute, even though she now had his brown eyes instead of her own green ones. Nice... David thought, smirking to himself as his cock hardened in his pants. It had been a subtle swap, but one that had got David aroused. And now that he'd had a taste...
His new eyes dropped down to the perky, B-cup breasts sitting under the girl's pink t-shirt, which was visible due to her open cardigan, and he licked his lips while pushing his glasses up the bridge of his nose. "Having boobs must feel nice." He said to himself quietly, concentrating again on the girl. David's cock throbbed once more as he watched the girl's feminine t-shirt flatten out as her boobs disappeared. Simultaneously, David felt a tightening in his own chest and he looked down as his t-shirt started inflating with the girl's former boobs. Oh god... that feels good... He thought, as his nipples enlarged, became more sensitive and became erect, shortly before the girl's white bra appeared on his torso to support his new 'girls'. He had to reach up and give them a quick squeeze, despite being out in public.
His penis, now fully erect, was constantly pulsating in pleasure as he studied his new chest, and he looked back in amusement at the nerdy girl, who now had his flat, slightly hairy chest hidden under her top.