Wow! Lucky you! Way to go
What are you worried about? How you gonna' "mess this up" ?? By not finding everything that we already don't know is there?
Just keep your detector set on no discrimination, dig every target, and be patient! Oh, yeah... fill your holes.

Any time I'm working on manicured lawn, (front yard, back yard) I carry an old rag tucked into the back of my apron. When I take out the plug and any other dirt, it goes on the rag. Then when I'm done, pick up the rag, dump all dirt back in the hole, replace plug, and stomp! Then I just fluff up the grass a bit - ta da! - you can't even tell. Keeping the dirt on the rag and off the surrounding grass keeps everything neat and may help keep your homeowner from getting freaked out over how many holes you're digging.
Remember... it's supposed to be fun!
And also remember.. we're all waiting for pictures!
How exciting. I don't know how you sleep at night. Good luck!
Kilika