"Without integration to external system"
Presumably you mean you don't want to store any of the external system data in SFDC, but just want to display some external content. .
Ok, you have options:
Custom Web Tab. This is a bit of overkill, but certainly, you could embed an external URL in a tab and have the external web page display as if it is appearing in SFDC. Maybe not the best idea in this instance.
Another option could be visualforce with an apex:iFrame component in it. This is maybe a better idea. You could either have that visualforce page render on its own. It then wouldn't be impossible to have the page linked to whatever data in SFDC that is going to store the acknowledgement.
I think there are options for custom buttons as well, but that's not something I've dabbled a lot in. But pretty easy to do where the button just renders in a second browser window. More difficult, however, is to store the response.
IMO, the Visuaforce option is going to be the most simple to implement and fits this requirement best.