The Arena Petrol is currently - to my knowledge - the national stadium but I believe this may change as the one in Ljubljana is undergoing a facelift.
Great ground with a large cineplex and bars/diners adjacent
Beer and burgers on sale inside the ground - believe the beer is only 2% though !
Golovec is main business and sport complex in Celje. Business part host many different fairs thru whole year. Sport complex contain open tennis courts, tennis halls, playground, and host hall for successful Celje Pivovarna Lasko handball team.
Mestni park is city park resort by the Savinja river. There you can find summer and winter ice skating place, trim track, tennis courts, bowling and so on.
It can be very crowdy there on fridays because many students come there to relax a little at the end of the week in schools.
Skating on the lake.
What can be better than that?
On a nice sunny day, meet some friends and take off to the lake.
Watch wrists though and dress warm :)
Equipment: Skates aren't essential, but if you want to skate around the lake I wouldn't recommend boots only.
Some people put kids on sleighs and wander around the lake.
Some even use dogs to pull them and if the ice is thick enough there are car races on the lake and it was also used as landing place for ultr-light planes.
There's a hall meant for ice skating and it's not that crowded. Unfortunately it's open only few hours per week - during winters only.
Equipment: Ice skates (they are availible at the checking, too)