Double check your antifreeze to see what temp its good for, a lot of people top off with water during the summer just because its convenient when your vehicle doesn't have an overflow, it dilutes your coolant so your freeze point raises, dryer sheets to prevent mice, start and idle for at least 30 minutes, anything less and your just creating condensation to make matters worse, and as far as additives, the only one I use and swear by is;
Seafoam, its used in the marine and off-road vehicles exclusively as they don't see a lot of winter use, and tend to sit for long periods of time regardless of what time of year it is. This stuff will prevent the fuel from varnishing in your carburetor, and will clean any out that may have already accumulated. I've poured this stuff in a motorcycle that sat for 12 years with old fuel and the carb was gummed up, changed the fuel, added the
Seafoam, started and ran like crap, an hour later and the silly thing smoothed out and ran like it had been run all along, I wouldn't have believed it unless I saw it myself. I refuse to use anything else, and you can dilute it as strong as you want,
Seafoam makes a few products, you just want the stuff you can buy at any automotive parts store for between 7-10 dollars that you pour directly into your fuel tank.