I understand it can be used as a replacement for nouns. Like for this sentence 物 is already being used for that purpose I think. So what exactly is の doing?

毎日同じ物を食べるのは、面白くない。

Thanks for any help.

In this case it is nominalizing the verb 食べる - making it into something like "the act of eating". You can't directly make the verb 食べる the subject of the sentence - 食べるは is ungrammatical. So you first turn 食べる into a noun with the nominalizing の, then it can be the subject. So you get:

"(the act of) eating the same thing every day, (it's) not interesting."

P.S. 物 is a noun, you're right, but it is serving as the object of the verb 食べる and not as the subject of the sentence.

Ahh, I understand now. It's been bugging me for a while...

Thank you so much for your help.

If you haven't read it yet the taekim grammar guide has a nice explanation for how and why it works in more detail in it's basic grammar section.

I haven't even studied the の particle yet for the most part outside of the possessive qualities it can be used for, but damn.

Delina pretty much made that explanation so clear and concise that even I could understand it (as well as the sentence). Kudos to you!