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!
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.
Friary Gardens is one of the three formal gardens in Cathays Park and the main feature is a statue of 3rd Marquess of Bute. The gardens were created in honour of the Marquess.