We stopped for lunch in Polychrono, nice looking pebble beach from what I recall, lots of restaurants and shops facing the beach (the road runs parallel to the beach for a fair way if I am thinking of the right place), and while I cant specifically remember seeing bike hire businesses, I wasnt looking, its a decent sized place so there will probably be quite a lot. No need to hire in advance at that time of year, wait until you get there.
I had a look at the weather underground website, if you select trip planner, put in Thessaloniki, Greece, you'll see the average temperatures from may 18 or June about 3 weeks later are 3deg C higher in June, and chance of a cloudy day drops from 22% in may to 11% in june, eg one out of 5 days likely to be cloudy in w/b May 18 compared to 1 in 10 w/b June 9th. This is of course historical data so its not a guarantee by any means but at least its showing you the odds. (FWIW by end of june chances of a cloudy day drop to about zero :-)