I've been to Hyeres many times and love it. You will want a full day on Porquerolles. My favourite beach is plage d'Argent, which is the nearest to the harbour (about 25 minutes' walk), but there are others. As there are no cars you walk or cycle so if you want to see the whole island it takes a little while to walk across.
You can also go to one of the other islands, Port Cros, from Port St Pierre. I haven't been there (yet) as I always end up on Porquerolles.
I think you could very easily spend another two days in the area, depending on which day you are there and what you are interested in. There are some lovely old streets, one of them covered over, in the old town and some fab food shops. If you are there on a Saturday, there is a very good market. If you climb up through the old mediaeval town there is a lot to see and it's a fairly stiff walk too. You arrive at the Villa Noailles http://www.villanoailles-hyeres.com/ If you keep climbing right up (take water with you for this climb) there is a ruined castle about 200m above sea level. There are only a few bits left but the view from up there is absolutely stunning on a clear day. The first time I went up there, it was June and the sky was full of clouds of enormous butterflies, two types of swallowtail and some others I didn't know.
Then there is the Giens peninsula with the little village of Giens on it. The peninsula is 5 miles long and has beaches along each side. There are salt pans near Giens and last time I was there I saw flamingoes. I am told I was lucky as they aren't resident there, but you never know your luck.
One thing to bear in mind if you are driving is that the car park at tour Fondue, where you get the boat to Porquerolles, gets very full in the summer so either get there early or take the bus from the bus station.
Hope this helps.