This might be one of the more obvious combos that there is (It has to be! I stumbled on it without being told about it or watching someone do it first.), but I haven’t seen anybody mention it, so I figured I would.
It’s interaction is immediately apparent. Hamlet lets you discard up to two (hopefully green, purple, or low value coin) from your hand, while providing you with useful +actions and +buys. Then Watchtower fills your hand back up.
Basically you trim your hand down to only the cards you really want, plus a watchtower or Library, and repeat, until you either don’t draw any more hamlets to discard the junk, or watchtowers/libraries to refill. If you’re running a deck with +$ cards, it’s sometimes possible to play them & discard down to nothing except a single watchtower in hand, and then fill back up & grab some more hamlets, and +$ cards.
It’s a pretty simple strategy, but at least in few games I've played them together it seemed very effective, especially if you can get the majority of the Hamlets.
This combo seems to strengthen some other kingdom cards that don’t give you extra cards, but give you coin, and especially +coin +action, (Festival, Minion might be interesting, Pawn?)
I’m just an average player, so I easily could be missing some way for an opponent to shut it down, or some flaw in my logic, I’d love to hear what the lvl 35+ guys think about this interaction.