I am a prospective university student.Recently I have been thinking about making money on the side to sustain myself financially in uni.Is there any concrete side hustle ideas.

Online gym training sounds like a feasible side hustle if it is an interest and passion of yours! It might be a little niche and difficult to acquisite a customer base if you don't have any credibility, so perhaps you can try starting out a YT workout channel to gain a following from there.

Perhaps you can also try searching for opportunities as a part-time instructor at workout studios, that way you need not start from scratch (not sure if you need certfication/experience for that though)

Sounds like a good idea aligned with your interest and skills! Could also consider starting a YouTube gym/workout channel, fitness youtubers have been gaining tons of views and revenue since the lockdown last year. Personally I have been watching and following more of them now.