In reply to DKS:
You’ll be thinking UKCers are out to knock back your ideas, but from our experience in early May some years ago you may find the Mercantour a bit limited that early in the season. Skiing had ended at Easter, but there was a lot of deep soggy snow around on the hills. Snowshoes would have been handy (and we didn’t have them). The higher roads were still blocked, and some of the valley roads, often in dramatic gorges, were being repaired or having rock-scaling done, so long hold-ups and diversions were quite common. And a lot of the facilities and even the accommodation were closed for between-season holidays and/or refurbishment. Once we adapted, it was certainly nice and quiet; the weather was pleasant if never exactly warm. We certainly weren't put off, and a later visit in June was much more like the thing - excellent high-level walking, fine scenery.
As an alternative, I can definitely recommend Mallorca in April and May, for walking, climbing, gorge-scrambling etc. …