I was given a simple way of doing this:

UPDATE BOOKINGS

SET DepartureDate =

DATEADD(day,

CASE WHEN Rand(CHECKSUM(NEWID())) BETWEEN 0 and 0.3 THEN 2 ELSE

CASE WHEN Rand(CHECKSUM(NEWID())) BETWEEN 0.3 and 0.5 THEN 3 ELSE

Round(Rand(CHECKSUM(NEWID())) * 28,0) END END,ArrivalDate)

Thanks for all your help.