What is a word for an activity without a concrete goal?
Solution 1:
Activities like you describe are called hobbies. Here's the definition from M-W:
a pursuit outside one's regular occupation engaged in especially for relaxation // Writing is just a hobby of his.
Solution 2:
Pastime
NOUN An activity that someone does regularly for enjoyment rather than work; a hobby.
'his favourite pastimes were shooting and golf'
ODD