Like Gardens, there are two ways to play Fairgrounds: either as consolation prizes because you missed out on a Province, or as a strategy unto itself.
If you treat them as consolation prizes, they are almost certainly going to end up being worth 4VP. It’s rare that you can’t get up to 10 unique cards; if you don’t have 10, you’ll probably know, because you’re running a very thin deck.
If you decide to go for them, you should realize that there’s only 17 different cards in an ordinary Province game, and 19 in a Colony game. So they’ll max out at 6VP each, after 15 unique cards. The way to keep track of your deck (as suggested by Blooki / Triceratops) is to glance over a Province board and pick out two cards you do not want in your deck, then focus on getting a copy of each of the rest by endgame. (Pick out four in a Platinum/Colony game, though going hardcore for 6VP Fairgrounds is much worse in a Colony game.)
Of course, these 17 & 19 numbers change with Alchemy (Potion), Young Witch (the Bane pile), Prizes, and Black Market. With those helper cards, you can pump Fairgrounds up to 8VP or possibly even 10VP each. Here, singleminded pursuit of Fairgrounds is much more viable, as they will end up paying off even greater than Provinces. But it takes much, much longer: a Fairgrounds deck’s critical weakness is how difficult it is to synthesize so many different cards together into a meaningful deck. Throwing in random crap from a Black Market deck is quite unlikely to work unless you are drowned in a surplus of +Actions.
This suggests, moreover, that the key to making Fairgrounds work is a set of non-terminals. In the late game, you don’t want to be passing up the last Fairgrounds because you need to get around to picking up a Moneylender. At the same time, you don’t want to just open with Explorer / Loan / Thief / Counting House, not unless you want a deck that goes nowhere fast. So ideally, you fit as many pieces as you can into a functioning engine (either because they are all non-terminals, or you have a ton of +Actions), and then grab the ill-fitting ones as close to the end as possible, so they don’t interfere with your Fairgrounds-buying engine. +Buy is golden here, since it allows you to get it done in far fewer turns than you’d otherwise need.
Like all Kingdom Victory cards, Fairgrounds does well with Hoard. Although it doesn’t do anything (like Nobles or Harem), its cost makes it a prime candidate for Remodeling, Salvaging, Apprenticing, and other trash-for-benefit Actions.
- Non-terminals, or a lot of +Actions
- Black Market
- Harvest / Menagerie / other cards that reward diversity
- Curse-givers (somewhat)
- Salvager / other trash-for-benefit cards
- Heavy trashing
- Terminals, with no +Actions