Extreme heat and high temperatures can be strenuous during Ljubljana summer and might make your days here dreary. You might wish to stay inside in the safe proximity of air conditioning and a refreshing smoothie with ice, if you are not adventurous enough to cool yourself down in many of the fountains across the center (this is in fact not forbidden by law).
However, if you are up for some movement on not-as-hot afternoons and evenings, and you prefer artificial climbing holds from those rocky ones built by the Romans, like the one renovated by Joze Plecnik and used by climbers in the very proximity (see article The Roman Pyramid) you might want to try out an outdoor climbing boulder wall located away from the city center in the Sport Park Svoboda on Vic.
It consists of two five meter high boulder towers connected with a boulder wall roof with many routes marked, but it’s possible to make your own depending on your skills. High trees around the wall protect the climbers from annoying sunbeams and the drink tap might save you from dehydration.
I suggest bringing your own climbing shoes or renting them by request from the Slovenian climbing portal Friko. Borrowing a crash pad from co-climbers is also a good idea, as falling from boulders is common and getting injured would not be a pleasant experience. After climbing you can chill in the neighboring bar with a glass of ice cold beer or other refreshments.