A polo shirt and whatever trousers you think are comfortable seems to be the norm; dark ones don't show the grime quite so much. Shorts? Not popular with most instructors and a hazard if you were ever to have a fire. But if you want to parade around showing off your hog bristle legs, that's up to you. Whatever it is you wear though, please make sure that it - and you - are clean. Sharing the close confines of a Spamcan with a student stinking of BO is not my idea of fun! Mind you, we once had a female student who doused herself in so much perfume that she was a toxic hazard in the aeroplane!
Quite often I find that I have to fly people whilst I'm wearing the 'day job' nomex flying suit and flying boots - not out of choice, I can assure you! Probably the most uncomfortable thing to wear in a Spamcan.
Track suits, yob-ball shirts or 'shell suits'? We'll assume you've come to collect the rubbish...