Mostnica Gorge
In early spring, when the highest peaks are still under the snow, it's time for other treasures. Korita Mostnice is for sure one of them. You'll be running upstream along the Mostnica - all the way up to its waterfall. The path is slowly ascending from the trailhead to the turnpoint, but there are no serious climbs. You should be able to run the whole time. On the way back, I suggest that you buy Bohinj Cheese at Planinska koča na Vojah - to carry some extra weight and to add some protein to your dinner.
There's an admission fee, which you can pay with cash. Check the details here
You need to pay for parking. I suggest that you come with coins, small banknotes, and a card as many times you'll be able to use only one of them (e.g., card not working).