Hoverspeed used to operate the hovercraft and Seacat services to Ostend. They have now ceased trading and there is no service for foot passengers wishing to travel to Ostend.
However, if you want to take your car, with up to 9 passengers, Transeuropa Ferries offer day return fares, from Ramsgate, at the same price as single fares. Until the end of March the fare is �35. During April, May and June it's �42. In the peak months of July and August it's �56, before falling back to �35 again in the autumn. (Those prices are for pre-booked travel. Booking on the day of travel incurs a �20 supplement).
As with any day trip, you don't get much time in Ostend. The outbound sailing leaves Ramsgate at 0700. As it's a conventional ferry, rather than a hovercraft or Seacat, the typical journey time is around 4 hours, although their slowest vessel (Larkspur) might take up to 5 hours. This means that you'd arrive between 1200 and 1300 local time. The return sailing is at 1800, arriving back at Ramsgate between 2100 and 2200.
http://www.transeuropaferries.co.uk
Chris