One idea was a workout mobile app with achievements (badges) - Duo Lingo for workouts. You pick a ready-made workout plan (or create your own) and it gives you badges to reward your progress and encourage you to keep going. What concerns me is that the market is over saturated - a quick search in Google Play shows hundreds of results for workout apps.

Another idea was improving the renting experience in London (where I'm based). There are so many new flat listings here that it's hard to find the ones you like. My idea is to crawl over the more popular agencies' websites and organize the listing information neatly with extra customizable filters, such as "1 mile away from a tube station" or "bedroom that is at least X sq.m." (for the ones that have an included floor plan). I haven't actually tried out that idea yet, and it would definitely solve a problem all my friends and I are having - we all go through a miserable experience in finding a flat. However, one problem would be that some of the agencies' ToS prohibit crawling.

I also had an idea about a Citymapper-like tourist guide mobile app, but with the pandemic, that won't be relevant anytime soon.

I’d say do the second one as it solves a problem you have. At the very least you get that. Also make sure you set up the business properly so that you are protected personally if the business does break ToS and you’ll learn that process as well.