This will depend on the season and climate of your destination. If any of your activities involve travelling to higher altitudes, make sure you bring layers regardless of how hot that country's climate may generally be. Jeans are not advised as they are restrictive and retain water for longer. Cotton shirts are best avoided too.
Instead, look for breathable, waterproof clothing with moisture-wicking technology. For moderate and challenging holidays, a sturdy pair of shoes designed specifically for longer walks is a no-brainer.
If you're travelling to a hot country, always come prepared with brimmed hats and lots of SPF.