Capture Selected Value from Drop Down List and Display Message

This might be a silly question and not really related to validation. But I am playing with adding new Display Messages to the application and I found that displaying

TempData["Message"] = entry.Date.ToString(); would successfuly display the date, but trying

TempData["Message"] = entry.Activity.Name; does not work.

If I want to capture the name of the Activity that the user selected (not ActivityId) on Controller and do things with it, how should I do that?

Any suggestion would be helpful!