The JW Marriott is a well designed hotel, and the restaurant has a good reputation - but it's miles from town. Probably 30 minutes in a taxi at a normal time of day, but I could imagine it taking closer to an hour in the evening rush.

The HGI is just on the edge of the French Quarter, and is in walking distance of the Old Quarter.

What you do on your morning before heading to the airport does depend on what day of the week it is. Many museums (including the mausoleum and the Temple of Literature) are shut on Mondays; a couple also on Fridays. I assume your tour will take you to the other Hanoi Hilton, the Temple of Literature and the Mausoleum complex, as well as around the Old Quarter, but if not then you could consider doing one or more of those. I've been rather lax on going to museums, but a friend had very good things to say about the Museum of Fine Arts, and the Women's Museum - the latter is quite close to the HGI.

Wandering the streets and drinking coffee is fun too, at least until you get fed up of the motorcycles or the heat (worth bearing in mind, it'll start getting hot soon).