My grandson is planning a honeymoon in February and wants to go to Stockholm. Would it be advisable - with winter are there venues open, limited hours, etc.?
About a million people live in Stockholm, so places will be open.
The drawback is that it is winter with very short days but if you are prepared for the cold weather than it is great since few other tourists are around.
Be sure they're prepared for not much sunlight. February has a monthly average of 75 hours of sunshine. I'd recommend comparing that number to the figure for the city where they're living today. Wikipedia may have that info in its city listing, in the Climate chart. I think of February in Washington DC as fairly grim, and we average over 151 hours of sunshine for the month.
As mentioned, there are people living there all year, so yes, things are in general open. Apart from Gröna Lund that is closed during the winter.
And February is at least a bit more than a month after the winter solstice so the days are not as short as in late December. Also remember that sunlight ≠ daylight.