It's been a few years (well, more than I care to remember) but my first trip to Paris (from Bristol) took me via Portsmouth, Le Havre, Caudebec-en-Caux and Rouen.
I stayed north of the Seine and took the direct route out of Le Havre to Lillebonne and Caudebec. The Seine estuary is quite pleasant once you're clear of the industrial parts of Le Havre though, and if covering the same route again I might be tempted to explore some more of the south bank - perhaps via the river ferry at Quillebeuf.
Rouen itself isn't hilly (most of it sits on the flood plain of the Seine), but there are climbs round about as the roads cross the river cliffs of the Seine and its tributaries. Look out for wiggles in the minor roads as they tack their way up the valley walls:
http://www.lib.utexas.edu/maps/ams/fran ... nm31-7.jpg
(That's a map from 1952. The topography won't have changed much, but some of the roads and river crossings have.)
If you're after a flat, direct, and almost traffic-free route out of Le Havre, the
Route de l'Estuaire that starts at the northern approach to the Pont de Normandie and follows the Seine between the river and the railway looks like a good bet as far as Tancarville. It looks as though you could cross the river there, then pick up another
chemin de halage on the south bank.
