When in Cardiff, a visit to Roath Park is a must! There is a huge lake with lots of swans, a children's park and flower gardens. A great place for chillin!
Next to St. Fagans is and an enchanting maze. In October I didn't expect to see such green landscape - but it is so green and lush.
Again Bute Park
This park close to the Cardiff Castle in the middle of the city also has some standing stones.
When the weather is nice, is beautiful.