Troubador Park has a jogging/cycling track along the bus lane. Great if you don't want to run along the footpath. Most local park their car at Mercury Pl.
Pearce Reserve which was recently renovated. IMHO the best part of Kings Langley, with small "river" flowing through to Banks Reserve, a tennis courts, a basketball court (still waiting for council to replace the backboard and net :-) ), a cricket ground, and one or two kids playground nearby. Most of the local event such as the annual Kings Langley Village Fair takes place here. The streets around this Reserve is best to live IMHO for family with kids or people who like to take an evening stroll.
Discovery Park has the biggest playground and quite interesting for young kids with curious mind. There are also two big parks: Endeavour Park and Morgan Power Reserve but haven't went there that often to comment.