Salcey Forest? There’s a picnic area, toilets, playground and forest walks, including one with an elevated section up to an observation platform.
It’s about 10mins from J15 (rather than 15A). You can then either work you way back to the A43 at Blisworth or Towcester, or head down to the M40 via Buckingham and Bicester.
May be a bit off your route, but it would certainly be possible to relieve some steam.
Otherwise it’s largely regular towns / villages along the route.
Alternatively, the services at J15a are almost on top of the canal (complete with a bunch of locks) if you just want to run backwards and forwards in a straight line…