Lets assume three weeks, which is a day or two to get over jet lag in Paris, a week in France beginning in Paris and ending crossing the Italian border, (for example from Grenoble to Turin on day 9), a week in Italy beginning in Turin and ending crossing the Slovenian border (for example from the Veneto region, not necessarily Venice, to Ljubljana on day 16), and four days in Slovenia ending on day 21 when you head back to Venice airport for a flight out on day 22.
Lots of possible day trips from Paris, Giverny, possibly all the way to St Michel, etc., and then on to the southeast Tours, Lyon, Dijon, Grenoble, etc. Rhone Valley vineyards and agritourismos possibly.
Turin, Milan, Verona, Padua, Venice, Bolzano, etc. Maybe south to Florence, or north to the Lakes Maggiore, Como, Garda, etc. We love the Dolomites Region in the Sud Tirol from Bolzano eastward to the Julian Alps in Slovenia. Last summer we did a driving tour for a week which was great, starting in Verona and ending back in Padua after visiting Slovenia and particularly the Trilev National park there. Lake Bled will be a zoo in July - as will Venice, and Paris.
You have year to plan. Check out the guidebooks as already mentioned to locate the greatest "second rate" cities of which there are many, and which preserve much more original charm than the "Top ten" destinations. . Check out guides for your favorite kinds of activities too, like hiking guides, bicycling guides, food tourism, etc. When we have booked major trips in the summer I start scoping it out during the fall/winter, and looking for bookings (with free cancellation) at least 4 months out, although I will not book all of them then (just the highest traffic spots). There will always be places, but not necessarily at prices you want to pay.
Have a great time!