I'd vote for just setting up a few proper footings for the posts, and using proper brackets to separate the post from the concrete.
Then you can choose the "flooring" you want for the pergola, grass, pavers, or a concrete pad and it can be easily repaired/replaced.
You do not want a wood post in the ground or in any concrete (or any contact with either), it will rot and be far more difficult to replace.
I do prefer brackets that use more than one mechanical fastener to the concrete and to the post (reduces twisting, stronger/more points of failure).
Figuring out the size of the hole/footing isn't to hard, look up or determine your frost line and soil type and then use online calculators.
Digging the hole, setting forms, and pouring the concrete doesn't have to be that hard either...it's fairly easy and cheap to rent equipment nowadays.