I think walking is quite relaxing in Paris as long as you stay out of the most touristy areas and it's not too hot. Or simply find a calm spot to read outside. =) The 14th and 15th as well as the less touristic parts of the 7th, the 10th and 11th arrondissements, should be less crowded and you'd be able to find some "hidden gems" there, like the observatoire de Paris. the 12th isen't that interesting perhaps, but continuing from porte dorée to towards the lake and parc on the way to the bois de vincennes is a great spot for a pic nic and a nice walk.
Other green areas that shouldn't be too overcrowded are buttes de chaumont, the forest around the former chateau de st cloud, bois de vincennes / bologne (but beware, certain areas of these places are to be avoided because they're mostly frequented by prostitutes and their customers, allthough I suppose the same is true for certain areas of any other parc as well, even the tuileries... =/). My favourite for a walk would be the promenade plantee, it makes you forget you're even in a city somehow. Also the below viaduc des arts makes for some of the most interesting shopping/window shopping in paris IMO.
The biggest market is the puces at clignancourt, but they are usually very crowded in summer and look out for pickpockets as well! In parts, it's more of a high end antique market than a traditional flea market. Various other markets appear frequently across the city in each arrondissemet. For food markets I would simply check the local info. I'm sure there are local markets especially for books too depending on where you stay.
Other than that you can find a number of second hand book shops in the area around the centre pompidou and the Gibert Jeunes at st michel square sell a mix of second hand and new books. =)
There are so many places to go to eat and as long as you avoid the most obvious spots, it's very easy to find great places for food in Paris.
Cafe visits that probably would not require 20 minutes in a line before getting in, I would recommend JP Hevin in rue st Honoré, prunier at Place Madéleine, le kong if only for the view, the restaurant at petit palais, any of the frères costes restaurants, l'altro in rue dragon...
In general I would go for a glass of wine and not the coffe as in general, the coffe is often not very good in Paris and dont expect them to know how to properly make any of the italian varieties that have made it to to the rest of the world (like if they serve a proper cappucino without whipped cream you're probably in a touristic place or in a place run by italians).
I hope you have a wonderful relaxing time in Paris. =)