Speak of angels . . .
I've been to Hyeres several times and don't drive so use public transport. Would suggest you stay in the town and take the buses along the Giens peninsula. See http://reseaumistral.com/horaires_ligne/ for local buses. Bus 67 is the one that goes to Tour Fondue from which you get the boat to Porquerolles. You cannot unfortunately get a bus along the western coast of the peninsula (or couldn't when I last looked) but you can get the bus to the village of Giens and walk round. I did this and was rewarded with a flock of flamingoes, though I'm told this was a fluke. This is the start of the 67 timetable reseaumistral.com/horaires_ligne/index.asp… you'll need to scroll to see the rest. As you'll notice, the buses are geared to getting the kids to school and their parents to work rather than for tourists so do make sure that you can get back to where you're staying by bus too.
There are other local buses (one goes up to near the castle I think, though I've always walked there myself).
It's very easy to get back to Marseille airport to get home. You take the train to Marseille St Charles, for the timetable see ter-sncf.com/Images/FH01-Marseille-Toulon-Hy…
and
the shuttle bus from Marseille St Charles rail station to the airport (E8.50, takes 25 minutes, runs about every 20 minutes)
or
further train to Vitrolles station plus five minute shuttle at the end.