You can not bluff until later in a freeroll. There are plenty of all-in donks as previously mentioned. Play solid hands and hope to hit some flops.
But, you must accumulate chips so you will / should play with a more aggression and less fear than a buy-in tourney, which equates to risking all your chips against the donks. You can make a little cash from freerolls, but there is much more luck involved in them with the style of play you will find there.
I think playing only freerolls could taint your style and lead to costly habits down the road. Put up a little dough and you will not only play in more enjoyable games, but your skills will better increase and the payout for a long grind will be far greater than the lesser tourney.
Good Luck
