Sites with hundreds of tools are ones I bookmark, but then never go back to, because I usually just search for what I need in the moment, it’s faster than finding a bookmark and hunting to see if a site I have as a tool for it. The only time this might change is if a site has an exceptional design or does things the basic top results don’t have that I really want/need, and it’s something I do often enough for it to matter.
I read a story not long ago about a guy who does pretty well with his converter site. It has a single tool, the URL is specific to that tool so it ranks high in the results, and it is something businesses need. This means they are willing to pay, because it saves them enough time to make it worth it, and they are coming back again and again. It wasn’t low hanging fruit though. If I remember correctly it could take in bank statements from bunch of different banks and parse them to pull out the details for some specific business purpose. When the banks changed their format, he’d have to tweak his code to account for it.
I see the age calculator is at the top of the popularity list, but a million sites can calculate an age. I just stick date it Wolfram Alpha myself. Most people probably also do this in their head when precision isn’t needed. I think you want to find what other sites don’t do, or do poorly, focus on that, and optimize to show up in search results for people looking for those things. If it’s something a business is willing to pay for, even better. You’d have to be hugely popular to make ads generate enough to matter, and it’s hard to stand out in the converter website market at this stage of the game.