Can you leave Versailles for lunch and then return?

The Paris Museum Pass (which is one of the things included in the 'Paris Pass' allows as many entries to a site as you wish. ANd if you only had a ticket for Versailles that day, you would still be able to visit the chateau then go out for lunch then return to visit the Trianon and Hameau (the other ticketed areas) The wider park is free and open without a ticket; the garden only requires a ticket on fountain days (and those tickets allow in and out during one day.

The Paris Pass is not a good choice IMHO but since you have it, make good use of the Museum Pass component.

Thanks Janet, and we will. :)

Janet is right, the gardens and other sites on the grounds can be done anytime. Depending on where your lunch in town is, you might find it more convenient to go back onto the grounds through the Queen's Gate. There is a more direct road to the Trianons and Hamlet from that entry. Then return through the formal gardens before departing. You can also pick up a pastry or snack in town to eat later in the gardens if it is a nice day.

FWIW there is at least one restaurant and also a kiosk where you can buy very good ice cream and various snacks and sandwiches at the base of the gardens just through the gate into the estate. https://janettravels.wordpress.com/2016/02/15/fountains-of-versailles-a-little-chaos/ They rent boats there, there is a restaurant and snack bar etc etc. so you don't have to leave Versailles and go into town to find food. I cannot speak for the food but the ice cream was unusually good to my surprise. We brought our own sandwiches from a bakery near the apartment we rented. We did not visit the chateau itself on this trip so there was no problem with carrying food.

The restaurant on the grounds near the Grand Canal is La Flotille and I had a lovely lunch there--house omelette full of yummy bits of ham, veggies and cheese--with frites. People nearby had what looked like pasta carbonara and also seemed appealing, menu offers various plat du jour type meals as well as sandwiches and burgers. It's been there since the 1890s. With all the walking you're likely to do might be easier to stay on the grounds.

If you do exit and want to re-enter for the Trianons and Marie Antoinette's estate (my favourite part of the whole experience) I second the recommendation of using the Queen's Gate entrance, I used it to exit and it seems a quicker approach than going through the grounds and gardens again.

Christa makes a good point. To see the estate requires an incredible amount of walking -- I would not want to walk back into Versailles for lunch and then back to the chateau and estate for more walking.

We've eaten at La Flotille many times, only because it's convenient and we wanted table service, but the food we've had was not very good. I agree it's a lot of added walking to go into town and back on foot. We rent bikes, right by La Flotille, and go all over the grounds by bike which is very fun and easy. Going into town by bike would be easy. Rental rates are very reasonable.