You can actually think an interesting application and generate your own data. For example, we were developing a product for processing network events in real-time. There were 6-10K events per second, and we were creating alerts for several different scenarios. For testing purposes, we actually wrote a program to simulate those events, with 20K events per second. It was generating fake (but realistic) data with the right format.

Application idea from top of my head: Generate turnstile data for different subway stations (enter/exit, time) and wrote an application to show the density of those stations with times. You can create a scenario where a certain station is more dense than others, and this could be your test. And this application could be your proof of concept