parks or playing fields in Hemel Hempstead

[Trophywife]

Morning everyone.

we're off up to hemel on Saturday, to see my Nan who lives in Adeyfiled, i was wondering if anyone on here lives there and/or can recommend a local park/ playing field, that we can take my Nan too for a picnic whilst keeping ds 3.5 amused and hopefully wearing him out so he sleeps on the way home (its a 4+ hour trip).

thanks in avdance