chimney climbing
Czech Republic is one of the very few if not the only country to have an official chimney climbing association - "Svaz českých komínářů" or the "Union of Czech Chimney Climbers" - a state-registered civic organization of people who climb factory chimneys and cooling towers as a leisure activity and also take part in industrial architecture history documentation as well as chimney maintenance and preservation. Post a message here to apply for membership: (http://kominari.cz/nkniha...).
pub crawl
There is a Pub Crawl that meets every night at 24 Dlouha street close to Old Town Square at 8:00 pm. It's cheap and they take you to some cool pubs, bars and you end up at a night club. Its a really good way to see what the Prague night life is really like. Even in the off season.
military adventures
Tank driving (http://www.offroadaction....)
Flights in the famous Czech-built Aero Vodochody L-39 Albatros jet trainer (http://www.migflug.com/je...)
geocaching
Geocaching is a popular sport in the Czech Republic, there are thousands of caches both in the cities and in the country. Czech caches are listed on geocaching.com, the descriptions are often bilingual Czech and English.
nudism/naturism
Although the Czech Republic is a land-locked country, it does have a lot of nudist/naturist beaches near lakes. A full list is available here: (http://www.naturista.cz/l...). Full nudity on other beaches is legal, but rare, and usually only happens in non-crowded places.
swimming
Many places in the Czech Republic are great for swimming, and there are many designated public swimming areas called koupaliÅ¡tÄ. A list of places suitable for swimming is available here: (http://www.plavcik.cz/). However, be aware that in hot weather the quality of the water in some places can fall below EU standard regulations.
hiking
Czech Republic has an excellent and sophisticated system of trail blazing, marked trails are about everywhere. Choose an area, buy a hiking map for the area best brand is "Klub Äeských turistů", 1:50000 military based maps covering the whole country, available in most large bookstores and go.Marked trails can also be seen on online maps (http://www.mapy.cz/#z=11&...).