Hiking or outdoor activities in May

There are some nice walks within easy train ride from Prague.
One I have done several times is Karstejn to Beroun.
http://www.praguepost.com/travel2/38261-from-karlstejn-to-beroun
You can take a train to Karlstejn from Prague (about 40 minutes) ,walk up to the castle in Karlstejn(15 -20 minutes) and have a look around for a bit then head out over the hills to Svaty Jan pod Skalou (st John under the rock.) this will take around 2.5 -3 hours walking from the castle.
The church there is a really nice and was undergoing refurbishment last time I was there 2 years ago. There is a small hotel where you can get refreshments and also a small place that is only open at weekends in the summertime.
You can then walk from there to Beroun which takes about 90 minutes or so.trains back from beroun run about every 30 minutes and takes about 50 minutes back to Prague.There is a really good Brew pub near the train station in Beroun called Berounsky Medved,set in the middle of a Scrap yard (I kid you not) but the food in there is not bad and the beer superb it is also very cheap.
http://www.berounskymedved.com/index.php/pivovar-a-pivovarska-restaurace-berounsky-medved-kontakt-a17

You could consider walking part of the Camino di Santiago in Spain. The weather is generally warm or hot, everything is open, and the routes are not as busy as the main summer months.

Also, May temperatures in southern France are moderate, and hiking/biking would be pleasant there then before things heat up. Bike tours in Provence are offered by companies such as Backroads.

Provence would be good in May, before it heats up in July/August.

Impassible trails are only an issue in the high Alps. Lower mountain ranges in much of Europe experience beautiful spring weather. Without going far from Prague, you could consider the Bohemian Forest, which crosses the German border as the Oberpfälzer Wald and Bavarian Forest. Or the amazing Franconian Switzerland region of northern Bavaria (despite the name, it looks nothing like Switzerland at all, but still beautiful).

Odd as it might sound, you could go north to Scotland. Spring is lovely time to walk in the highlands.

Pam

Saxon/Bohemian Switzerland is one of Europes best hiking areas... and just 2 hours away from Prague. May is the perfect time, and if the weather is bad you can take the paddle steamer to Dresden.
Here is a site with hundreds of hikes (+ pics)

They reopened a hike near Malaga, Spain long considered the most dangerous walking paths.

http://www.theguardian.com/travel/2015/mar/27/caminito-del-rey-malaga-path-reopens-spain-hiking

Thanks everyone for your responses - much appreciated.

What Martin said.