We included the stamps in my fiancé's passport (USC) but I didn't have any stamps in my passport so we included my CBP travel history which you get online.
To back all this up we included air bnb confirmations, flight related information and car rental information.
We also included moral statements - a couple of letters from people willing to swear they have seen us in person and we have stated our wish to be married etc.
Out Statements for intent to marry were separate and both notarised. They were both a bit different in the fact that my fiancé kept his short and to the point like the one that has been shared before.
I however included some information about how our relationship started, key moments from the point like first meeting, meeting family, visits, our daughters birth.. and what I think is more likely to occur - for example
"If a k-1 Visa is granted to me, we intend to marry within the 90 days after I enter the United States. I would just want to make sure my immediate family could be there for the ceremony as it's important to both of us that out parents and other family members be present and they look forwards to seeing us married. I would get myself and my daughters dresses in the UK so that my family and friends can be part of something in case they cannot make it over to the US in the 90 day timeframe. We will do a simple ceremony to save money and make it easier to have a more flexible date considering all of his."
You can 100% keep it simple, but I had read giving a little more information in a notarised statement can be useful. It probably depends on circumstances - but we didn't get any RFE's and it's now at the NVC.