Hyeres is a very spread out sort of place. The town centre is about 15 minutes' walk up a gentle-ish slope. It's on the edge of the mediaeval bit. Then beyond and behind the town centre, the old town stretches up a steep hill. If you go far enough, right to the top there is a lovely garden and the Villa Noailles and above that a ruined castle - I'm not sure how accessible that is, it is a couple of years since I went up there. One way to do it would be taxi from the station up to the Villa Noailles then walk down through the gardens and see the old town that way.
Then there's the rest. Hyeres is set very slightly inland, with the Giens peninsula running from the south of it to the Tour
Fondue, from which you can get boats to Porquerolles, the idyllic island off the end. Boat trip about 20-30 minutes and you're in heaven. White sand beaches, pine trees, no traffic. There are buses along the peninsula which is five miles long so not for walking unless you are VERY keen.
Here's the timetable for the bus 67 that goes from the station to Hyeres town centre one direction and the Tour Fondue in the other. http://www.sodetrav.fr/pdf/67.pdf I would check when you arrive that this timetable is correct - it may have changed by then.