I have an annual meeting in Cardiff that lasts the morning but I tell work lasts the day. I spend the afternoon at Cwmcarn. I considered Cwmcarn near Cardiff but I guess it depends on context of how far you are travelling.
Yeah Cwmcarn is about 30 mins in good traffic, it’s nice TC so sign posted, all weather surface etc – perfect for OPs needs. As is BPW which is a bit closer for most of the City.
There’s loads of great riding to the North, as Taxi says Tongwynlais is pretty much the hub of it, but there are no office MTB trails in Cardiff and you could easily spend all day trying to find something you like.