Haarlem is close enough to reach without real transit planning, and small enough that a day trip does not turn into another day of walking a marathon didn't already cover.
Grote Markt
The main square is dominated by the Grote Kerk and ringed by cafe terraces, all within a few minutes of the station. There is no obligation to see anything specific here: sitting down with a coffee in view of the church facade is a legitimate way to spend an hour on recovering legs.
The Frans Hals Museum
Housed in a former almshouse, the collection of civic guard portraits gives a sense of Haarlem's Golden Age wealth without the crowds or scale of an Amsterdam museum. Manageable in under two hours.
Where to Stay and Eat
Haarlem works better as a day trip than an overnight stay for this itinerary, since the marathon's own hotel base in Amsterdam is a 15-minute train ride away. Café terraces around the Grote Markt and along Zijlstraat cover most casual eating needs.
Getting Back
Trains to Amsterdam Centraal run every 10 to 15 minutes, keeping the return leg as short as the outbound one.