Being gifted with an old run down estate with a castle as a reward for finishing the main quest.
You can spend your gold on rebuilding it and adding shops + an inn, recruit bards, servants and shopkeepers to work there.
Send your friends/companions there to hang out or work as guards while your husband/wife takes care of all the day to day business and finance.
If you ever played NWN2 then Crossroads keep might spring to mind, it had potential but was never really realised, I think something like that would fit well into this game but it is hardly a little thing to add.
Edit: A stable for your horses too.