For Crown, i think the better solution is a Reaction that triggers at the start of your buy phase, as mentioned before.
What's wrong with an action-treasure?
The main complaint I've heard is "It's too hard to remember whether you played it as an action or a treasure" Dominion has a lot of memory. I don't think duration are stupid because you have to leave them out. Mining village isn't stupid because if you trash it you can forget how many actions you have. Seasons is a great expansion even though it's easy to forget to move the marker.
EDIT: 100th respect! Thx AJD!