I'd spend the whole day at Versailles. The Chateau is THE palace in Europe (but it will be ridiculously crowded. and hot), so it's definitely worth seeing at least once. The gardens, however, are where it's at! I've been to Versailles 5 or 6 times and I've been in the Chateau only once (I'd do it again but my wife has no interest). But we often spend entire days in the gardens and the Trianons. M.A.'s hamlet is one of my favorite places to visit and you can rent boats in the grand canal, rent bikes to ride through the gardens, sit at cafes to sip drinks, walk among the many fountains (if they're running - depends), the orangery and hedge rows. A full day is easy and I think should be planned for.
However, if you see all that you need to and still have time in the day, I'd train it back into Paris. You can spend the summer there and not see it all. No need to look around for anything else outside of Paris, you'll have plenty to do in the city. Even if its just walking a people watching, Paris ranks highly!