Agree with all. Add fresh content! I believe it encourages indexing as well.
Interesting about changing the post date. I hide most of my post dates visually, but I figured G already knows the REAL date my content was published. I am sure they store a date/timestamp in their huge databases. I just don't want people to see "old" content - so no date better than an old one.
Adding content will keep your website "fresh" but for rankings you still need links. Whoever says that only content matters doesn`t know anything. If you`re using WP you can always keep the content fresh with a plugin which changes the post date (it will increase CTR from search results too cause people want to see fresh results) but adding new content is also necessary.
Cheers