The sim instructor likes to fail both FMC's just before a turn in the airway, so fly the aircraft using heading select and try not to get too distracted as PF.
Autothrottles are gone too, so beware if it happens near a level off during descent.
IRS and GPS navigation is still there, it is your database that is lost. So if already in oceanic airspace, why not continue instead of turning around(entering oceanic may be a different matter). You already have your flightplan in the FMC which is still there but creating a new flightplan is time consuming doing it the old fashioned way with lat/long. But maybe a route copy is good for destination in case you go around and want to have your approach waypoints still available.
I have to admit, when practicing it in the sim, I remember that some waypoints seemed to not be connected anymore. Anyone know more about this?
Only Nav/Rad, Prog, and Legs pages are available but no fuel prediction.
Anyways, aside from following the QRH for stuff like manual pressurization, go to the just go to the performance inflight and get you planned Vref. Then on a piece of paper, write up a landing data card like in the old days with min maneuver speeds being Vref 30+80, 60, 40, 20, and 10 for flaps up, 1, 5, 10, and 20 and then of course mark down Vref and it is all easily referenced plus go-around N1.
Vectors is best but in the sim you may have to manually tune VORs for arrival and manually tune and identify the ILS. And of course no VNAV performance.
Probably some other stuff as well.