Don't know if I'll be able to make it, but I live in the area and have some advice:
Generally in the summer it's impossible to park in Old Montreal. Especially on fireworks nights. It's a tourist area, so people are either looking around or dodging pedestrians and then you get stuck behind a caleche (horse-drawn cart).
- Try to avoid driving any closer to the water than Notre-Dame, since De La Commune and Saint Paul can get insane. Especially between Berri and Saint-Laurent. During the fireworks (10-10:30pm) traffic on delaCommune stops.
- If you want to try parking for free, try around Saint-Antoine and Viger (these are the east/west roads straddling the villemarie) around Saint-Laurent or near the Palais de Congres .
- If you're willing to pay then the Quais have parking (don't know how much), but then make sure you don't spend time on Saint Paul or delaCommune with your car. There's a parking lot at Saint-Antoine and Saint-Laurent. There's one near Place Bonaventure and there's one west of Saint-Laurent with entrances on Notre-Dame and Saint-Jacques.
- There's very little unmetered parking that is for non-residents. A couple of metered spots turn into valet parking (or débarcadere) at 6pm, these tend to be in front of restaurants.
- The parking jerks pass often.
My suggestion: metro/walk or bike in. The champ-de-mars station is closer. Also, there are plenty of Bixi (bike-taxi) stands. $5 gives you unlimited 1/2 hour bike rides for 24 hours.
If you're looking to fill some time up, walk around the old port, check out the clock tower and a quai or two, Place Jacques-Cartier and city hall. Also some hotels/restaurants have rooftop bars (tend to be expensive but nice views).