Thanks a lot for all the encouraging words, everyone! I'm glad you like the mechanic.
Hidden Passage is pretty interesting as well. It actually provides actions which can be used to activate itself. If you do activate it right away, it's pretty much a non-terminal Secret Chamber, which seems reasonable for $3. But if you wait before using the activation, it can actually get close to Vault in power. If you wait until the next turn, you can discard 5 cards instead of just the 4 you'd get from Secret Chamber. Not quite as good as Vault, which discards 6, but there also isn't any benefit to you opponents. Moreover, Hidden Passage combos better with other cards because it is non-terminal. A key difference between Hidden Passage and a non-terminal SC is that you are more easily able to play draw cards before discarding for coin. You could play Hidden Passage and then Smithy before activating, a trick that wouldn't be possible with a non-terminal SC without some other village support. I think $3 is still a good price for it, but it is more powerful than it first appears.
I am incredibly pleased that you 100% nailed the concept of the card, including the "Play Hidden Passage, play Smithy, activate Hidden Passage" combo.
Boomtown is very neat in that the activation is a penalty which must be paid if you ever want to play that card again. This kind of penalty is really interesting -- you can wait to pay it at a time where it hurts less (in this case, waiting until you have a junk card in hand and actions to spare), but you need to be wary of missing the reshuffle. I think Boomtown is nice as is, but other cards could have an activation that is even milder than "discard a card". Even if the activation efffect was entirely neutral, it could still be a penalty in that it uses up an action.
Funny enough, I was considering this for Boomtown. Right before I posted this thread, I had penciled in "something weak" for its activation effect. I thought about just +1 Card or +1 Buy, etc. Then I thought of just, "When you activate this, do nothing." But then I (luckily) did GeoLib's sanity check and realized that would make it strictly better than Lab at $5. So I posted the version you see above.
I think an interesting activation penalty would be "discard your hand and draw 5 new cards". It is neutral on the surface (minus the action cost), but can be used to your benefit to mulligan a bad hand. However, waiting on activation can cause the power card to miss the reshuffle, a danger which is made more likely because it draws cards itself.
Huh. I would consider that to be a bonus way more often than a penalty, although the lack of +1 Action somewhat limits its usefulness. I am definitely interested in making a card with a mulligan effect. The problem is making it not suck when you don't want to mulligan. At one point I was considering this card:
Corridor
Types: Action
Cost: $3
+1 Action. +$1. Look at a card from your deck per card in your hand. Either discard those cards or discard your hand and put them into your hand.
I never actually printed it for testing, though. Knowing what I know now, it would be a bear to resolve with larger hand sizes. Right now I have Terrace in Enterprise:
Terrace
Types: Action
Cost: $4
+1 Card. +2 Actions. You may spend a Trade token. If you do, discard your hand and +5 Cards.
When you gain this, take a Trade token.
It's working out great so far, so I already have one good mulligan card. I really the idea of an Activation mulligan, though. It ensures that you have it when you need it, like Balcony does for playing a card twice. Chances are that if I make such a card, it will be different enough from Terrace that I can do both.
As for Patron being worth more than $2, perhaps that's true. I take that criticism with a grain of salt, though. Often it seems that any $2 card worth buying seems "too powerful for $2" to most people. There's this perception that $2 cards need to be weak, but that's a fallacy. Cards that are weak ideally don't get published at all. In playtesting, they don't get bought.
Anyhow, the upshot is that I'll probably test it at $2 first and keep an open mind. Either I am overestimating or several others are underestimating how harsh having a double-terminal is. Really I think it'll come down to how useful it is to just leave them in play until game end. I'm hoping that the +$4 will be enough to entice players to sometimes activate them even in the face of Attacks being played.
In other news, I've been trying to come up with a good Activation Attack. The challenge is that, ideally, the attack effect should be blockable with Moat, etc. So I'd rather not do a card where the attack itself is on the Activation portion. Here's an idea that uses an ancillary Activation card (because I'm a big proponent of Attack cards using ancillary cards).
Cannoneer
Types: Action – Attack
Cost: $4
+1 Action. Activate all your deactivated Cannons. For each one, each other player trashes the top card of his deck and gains a cheaper card that shares a type with it. If he didn't, he gains a Curse.
When you gain this, gain a Cannon.
Cannon
Types: Action – Activation
Cost: $0*
+2 Cards.
When you activate this, +1 Card. (This is not in the Supply.)
Maybe a bit wordy. Still, seems promising as a mechanic.