Datum and agendum are singular. One singular piece of data and one singular thing to do, rather than a list. The minute it is two (or more) they become data and agenda. So, "What's on the agenda?" is correct being people are going to expect more than one item. If I say the only thing on the agenda is the meeting, that's wrong, it's the agendum. Yes, I am just being a grammar nazi. I don't really care that much about those words.
[INJ} I am given to understand by today's youth that "to bail" would indicate giving up in a dramatic way. It would appear to come from "bailing out (of an aeroplane)". Incidentally, "Keep Calm and Carry On" is available on a mug in my own kitchen.
[Kagome...] If you have a lst of things to be done (from the latin gerundive), or an agenda, and it only contains one item, it is still a list of things to be done. So I disagree, and would say that an agenda for a meeting is always an agenda, regardless of how many items are on it. More specifically, each item on an agenda would be an agendum.