In summer I wear just a diaper and plastic pants. I might put a T-shirt on during particularly cold nights but throughout my life I've usually liked to sleep in as little clothing as possible and this has continued through untraining.
In winter I'll usually wear one-piece pyjamas. I've built up quite an impressive (I think) collection of reasonably non-babyish one-piece nightwear, although I realise that one-piece pyjamas kind of look babyish by definition - just a risk of the trade, I guess. If I need to be really discreet, I'll wear normal two-piece pyjamas with a onesie underneath them - it looks odd but not usually odd enough to raise questions.