From my experience
Hydration: nasal spray called "Sterimar" which is a salt water spray (no drugs) is very good for general hydration and unblocking of nasal passages.
Colds: Sterimar is one of only three things I have found to help reduce the time a cold blocks you up. The other two being Olbas Oil (which causes irritation if used too much, so be careful) and a Steroid given to me by an AME once. Not recommended for general use and his prescription was controversial.
Eye drops: You can get a brand of lubricating eye drops called "Blink" which are designed for contact lens wearers. The great power of these is that they come in single dose containers, about 30 to a pack. They are easy to carry airside and do not have the disadvantage of having to be chucked away after a set time due to bacterial build up. I tend to carry one or two doses (5ml ish) in my pocket for night flights and early mornings! They also help if you have allergies - sometimes when I rub my eyes (I know I shouldn't) they get irritated - Blink helps clear the irritation, although this is not what they are for. I don't use them often, but they are refreshing and helpful especially on long flights.