Lots!
OS map is your friend here
There is a old rail track running in one direction to St Albans and the other to Welwyn and onto Hertford. Lee Valley canal at the end of that
Broxbourne Woods isn’t too far away, but there’s lots of tracks and paths for a CX type ride
Also another track called the Ayot Greenway, runs from Welwyn, that takes you into a lovely part of Hertfordshire
Look for the Rapha Hell of the North rides (7th or 8th installment this year), as they’re rides that would be perfect for your CX bike
Where is your assignment and what time are you going? Traffic can be hellish round there at certain times (I used to work near there). Within 20-30 min you have places like St Albans, Harpenden, Hertford, Stevenage (this last place is very average). This will all let you explore a different part
There’s nothing gnar around, but there’s lots of cycling. A lot of the roadies and CX groups will head up that way from Central London