Just to elaborate on the main paragraph where I have another perspective on this:
I assume that they are expecting tangible results, something "out of
Not necessarily. Software development can cover a lot of different possible roles as some developers may double as administrators, analysts and managers of various items. There can be database, network and system administration to be done. There can be business analysis, functional analysis, system analysis and probably a few others. Thus there is the question of what did and didn't you have to handle in your previous role. In some jobs, I had to put new computers onto the LAN that may or may not be what people would expect from a developer.
"my boss/client said I did a good job" (cannot prove it),
This is quite vague and thus would be a poor answer as there isn't anything really new added as I doubt you'd say you met expectations, right? Now, if you changed this to be, "The client for whom I built an e-commerce site that can handle thousands of transactions a minute using ASP.Net, C# and an MS-SQL Server back-end," then this gives a bit more of what you have used which can also be something to notice. Are you proud or not of what you have used in the past? Some people may well change their voice when saying various technologies or tools since they have various feelings about them.
"I fixed a bug that was very hard to debug" (it was your job anyway)
If you could give more of a perspective on that bug, it may be a good story here. I can think of various bugs where developers spent days on it and I came in and fixed it in a few hours with some help from on-line resources to help me learn about X that helped me patch Y system. While it is part of the job, it could be a worthwhile piece that was done.
"I enhanced an internal tool of the company even when I wasn't asked
to do so" (doesn't sound so impressive).
So, taking an internal tool that was barely functional and adding unit tests, making it meet coding standards and adding documentation so that it could be more easily maintained, isn't that impressive to your mind? There is something to be said for what details you add that can make even something as simple as "going to the store" sound like a Quentin Tarantino film.
Also, while I'd love to mention something like "I gave a talk on so
and so", I never had the chance (basically because I don't think I'm
an expert on any subject yet).
Do you really believe you couldn't present a talk on a subject of something you've done but haven't mastered? In some development places there can be demonstrations of a system that usually are done while it is still being built. Those builders are wanting feedback as sometimes it is more important to know whether or not something is going on the right track. Consider how you posted this question and may well now have seen much more that can be done.