I've done the overnight ferry AMS TO UK a few times and liked it. Check out the guy in Seat61 www.seat61.com. There are great suggestions.

With your plans, while it is faster to fly across, think about the airport hassle. For coming home, Paris to London is on,y 1 hour, so really shouldn't be bad.

For Paris, a lot depends on what you want to see. You might consider taking a day tour to hit the high spots, which you can always return to.