Q: SAMUEL CLEMENS' POCKETWATCH: When the pocket watch says "One action that must happen on your next turn (such as your card draw) happens now instead." Is that just the card draw, or can it be something else?
A: It means anything which is "scheduled" to happen next turn as a result of the instruction from a card, or from the rules (such as the card draw). The key word on the card is MUST. MUST is not equal to MAY. There are many things you MAY do next turn, but only a few things you MUST do. If you're trying to take an action with the Pocketwatch, ask yourself, "Must this happen next turn?" If the answer is yes, you can do it. If the answer is no, you can't.
Here are some examples of uses (and non-uses) of the Pocketwatch (SCP):
Showing a Devidian Door -- Since you MUST show the Door next turn, you can use SCP to show the Door immediately, so your opponent has no chance to get it out of your hand until your next turn. Note that you could not use SCP on your opponent to make him show a Devidian Door, because you can only use SCP on yourself.
Playing an Event -- Are you required to play an Event on your next turn? No. This is something you MAY do, so you cannot use SCP.
Time Effects (Temporal Rifts, diseases, Anti-Time Anomaly) -- If this is the next-to-last turn until these effects resolve, then at the end of your next turn, they MUST resolve. Therefore, you can use SCP to "speed up" the process.
Cytherians -- If your ship is under the influence of the Cytherians, you MUST move it next turn, so you could, in this case, use SCP to speed it along (keep in mind that this replaces the actual act of moving it next turn, so you could not move it next turn -- it used that turn's move when you used SCP).
Borg Ship -- The Borg Ship MUST move one space down the spaceline on your next turn, so you could move it an extra space this turn with SCP. As above, it would not move its normal space at the end of your next turn, as it already used that movement when you used SCP.