A hobby is a harrastus, and the verb harrastaa — to do as a hobby — takes the partitive: Harrastan urheilua. — I do sport.

Finnish	English	Finnish	English
lukeminen	reading	urheilu	sport
uiminen	swimming	hiihto	cross-country skiing
matkustaminen	travelling	pyöräily	cycling
ruoanlaitto	cooking	musiikki	music
saunominen	going to the sauna	marjastus	berry-picking

Mitä harrastat? is the standard way to ask about someone's free time — Finns ask it the way the British ask What do you do at the weekend?