When I was touring, I was in Italy during May and it couldn't have been more gorgeous. Sunny weather, beautiful scenery, fun cycling, lots of art and culture and history and food. There's some tough terrain in spots, but I think those spots are easily avoided, especially if you do a self-guided tour through a company.

England was fun, too--very accessible with lots of varied terrain. I was there July through October and lucked out with the weather. Ireland would also be good for cycling, I think. I was just there last month and was wishing I was on a bike. We saw a ton of cycle tourists, too, so you'd probably be in good company.

I was the rare cyclist who was NOT a fan of cycling in the Netherlands. Belgium was very nice, but Holland was a) boring (as far as the cycling goes--flat with headwinds wears me out) and b) sandy. My main memories of Holland (besides cold rain, but I was there in November, so that's not fair to compare with May) involve spending inordinate amounts of time removing grit from my equipment.

Have fun planning! I think that overall you can't go wrong--May is likely to be a good time to cycle pretty much anywhere.

Sarah