This depends on what you are doing. Let's start with some caveats:
1) Speed is important - you don't want to spend a lot of time screwing around, you just need some traffic
2) Money is important - you would rather have some people on the cheap than a very well qualified audience that you paid more for.
3) You are pretty early - you don't have some massive social media following and stuff like that. Frankly, if you had that you would already be getting 50 users per day.

Assuming these caveats are true, people suggesting things like Google/social media/etc. are probably heading you in the wrong direction. You probably don't want to spend days and days hunting around Google for pockets of low CPC keywords that have enough volume to get you the traffic you need. You probably want to just dial it up, set it and forget it.

If #2 is not true, then Google Adwords is solid. You will pay a pretty high CPC, but you will get a very well qualified, bottom-of-the-funnel kind of audience.

If both caveats are true, I would concur with other people that mention Facebook. Big audience + Low CPCs = easy to get 50 people per day.

Things like "advertise on some podcast", "start a blog" or "get some press" seem like advertising mechanisms where you cannot control the amount of traffic you generate on any given day, may be below your target or above your max, result in spikey traffic, and may take a long time to actually get started. I wouldn't do that for testing.

Now, I have no idea what you are doing, so the true, true answer may have some variability based on that, but these general guidelines are pretty good.