The road leading away from Arc de Triomf connects to Ciutadella Park which is a leisurely 15 min walk. It has a beautiful fountain and other sights. More info here. This park is a lovely green space to walk around and enjoy the surroundings. Unfortunately we went on a really hot day and had to hot foot it out! Ice creams were literally melting between bites!
Barcelona is full of green parks which is a must for me in a big city. One of them I visit very often, Ciutadella, because it has a wonderful fountain and a lake. But what differentiates it from all the other parks is a statue of a life-size mammoth next to the lake. Apart from clicking pictures of this huge mammoth, this park also has a zoo to visit. There are beautiful lakes and museums inside to visit too. A great place to relax and learn!