Excellent idea! Overnight ferry out, overnight ferry back - a whole day on the other side. Train to ferry transfer is very easy on both sides, the only drawback being that if you do the trip on a weekday the journey from Hoek van Holland to Rotterdam will be on a packed (and slow) commuter train. Stena Line positively encourages foot passengers, and sells competitively-priced "Any Dutch Station" train tickets as part of the package.