This is a great question. It is also an impossible question, because nobody knows. We don't get to see the girls in the forest. We only get to see and hear what they say happened, and what the others conclude.
We know Parris says he saw a kettle. We know that the girls (in Act I) first say they just did "common dancing," and then change stories to say that Tituba called the devil. Tituba denies that she called the devil, but then changes stories to say that there were witches there. The girls then change their stories to say they saw women from the town with the devil.
What happened? Do you believe them under pressure? Were they playing around? Running a scam? Possessed? We have only their testimony to go on, and so we can't be sure.
It is true that we don't know exactly what the girls were doing in the woods, but we do know that they were surprised by Rev. Parris, and it is this event that triggers the strange "disease" that Parris's daughter Betty will suffer from.
Actually, we do know what they are doing. The girls in the woods were dancing naked, without any clothes on and they were eventually discovered by Reverend Parris, who had seen Tituba “waving her arms over the fire” and muttering some sort of gibberish and nonsensical mantra from her mouth over some sort of cauldron, which we later discover that she was experimenting with it, to forced the souls of the Putnum’s dead babies to come out of their resting places. Abigail then drink the blood of a chicken to put a terrible curse on her former employer, Elizabeth Proctor. They were actually practicing some sort of witchcraft, chanting curses and spells to perform dark magics.
I think they were involving with witchcraft when they were out in the woods