The Czech Republic has a great and complex trail blazing system, with designated trails almost everywhere. Choose a hiking region, get a hiking map (the finest brand is “Klub eskch turist,” 1:50000 military-based maps covering the whole nation, available in most major bookshops), and go.
Many locations in the Czech Republic are ideal for swimming, and there are many approved public swimming spots (known as koupalit). A list of swimmable locations may be found here: [www]. However, keep in mind that in hot weather, the quality of the water in certain areas may fall below EU standards.
Despite being a landlocked nation, the Czech Republic has several nudist/naturist beaches around lakes. A complete list may be found at [www]. Full nudity is allowed on other beaches, although it is uncommon and generally occurs in less-crowded areas.