Some ideas I've had are:
* Website templates (i.e WordPress). Downside: Will require constant maintenance to stay up to date with latest versions, bug free, and, secure
* Apps. Downside: Very competitive field
* Very NICHE apps.
This last point, is what I think would be really useful. For example, there's a sync for Fitbit / Apple Health. The guy who made it, has made two different apps, one for each direction, and they both cost $7 each. If you want to sync FROM Fitbit to Apple Health, that's $7. The other direction? Another $7. This app does it well and because it's such a niche thing - only for Fitbit and Android users who want their data into Apple Health or from Apple Health, it seems to work.
The problem here is finding these niche areas where there's a void to fill - these apps are so simple because it's basically just 2 UI controls (to/from date, and a button) and then two API calls.
I'm now trying to think of these things whenever I end up in a situation where I felt "damn, other people must want this too, why is there no simple app that does this?"