Bocat - I cycle through Vale de Parra quite a lot. It is generally flat and you could easily use a bike to get to the beaches at Gale, Salgados and Armacao de Pera. Albufeira is also OK but there is a very steep hill of about half a mile to climb on the way or you can do as I do and go through the marina and up the old road which is less steep but not well signed. You can also cut over to Guia and Algoz and do a bit of exploring without too much effort. Vale de Parra is a couple of 2km from the seaside, so I wouldn't exactly call it a resort. There are a number of bars and restaurants however for the evenings.
I use the side roads that run parallel to the main roads where ever possible. Watch out for potholes and sharp stones.
However I am racking my brains to think from where you can hire a bike. The one place I knew of in town has closed down. I will keep my eyes open.
Weather in September will be like it is now in June - 80's during the day and mid-60's at night. However around the end of September the weather breaks and for the last two years we have had massive storms and floods. You should be Ok in the middle of the month.
Hope that helps.
