Our base for our stay was the Tokyo Metropolitan Hotel at Tokyo Haneda. This new 4* hotel is located close to Terminal 3 and was 50% cheaper than the Royal Park or Villa Fontaine Grand hotels which are next to T3 Rooms between 23 and 31 on floors 8, 9 or 10 give good views of movements but you need to be quick to get the departures from 04/22 so rooms 23/24/25 are probably the best due to the better angle. These rooms are known as the airport view rooms and have a small premium (<£6 per night).
In addition to the obvious spotting from the room option there is a roof top terrace which overlooks the airport which I'm sure you could stay on all day but as the viewing deck at T3 is so close in my view there isn't much point in doing so.To get there you need to head for the Tenkubashi station, cost <£1 from the airport, on either the Monorail or Kekio line. Close by is the Keikyu Ex Inn which is on the other side of the river and evidently also gives airport views.
The hotel was booked via Bookings.com and were understanding when we had to request a cancellation for our first nights pre paid booking at the last moment due to being unable to get on the direct HND flight from LHR.