The Warner Bros. Studio Tour at Leavesden is the only thing in Watford with international pull, and it deserves the reputation — book months ahead in summer. Beyond that, the appeal is local: a vast Edwardian park along the River Gade, a working Edwardian theatre, a football ground that punches above its weight, and a few quiet historical corners that survive the busy shopping centre.
The town is easy to do as a half-day from London — twenty minutes on the fast train from Euston into Watford Junction — and combines well with a Studio Tour booking. Don't expect medieval England; Watford was a small market town until the railway arrived in 1837 and grew up around printing, brewing, and commuters.