Water bottle and a credit card.All the tools you will need.
2 or 3 days you can bring 1 change of outer clothes. Remember you can wear the same outer clothes more than 1 day. Most anything you might need unexpectedly can be bought along the way. Bring a water bottle and a cell phone. 500 miles is not a big trip, Don't over think it. Just have a goof time. This is how my wife and I took long trips. We went from So Cal to Portland Maine.
Water & sunscreen are #1 & #2.
And carry at least one tank-full of cash. One of my first longer rides, I ended up on fumes in western Minnesota. I stopped at a Farmer's Co-Op to fill up, but they didn't take credit cards & I had no cash. I told the little old lady behind the counter, "I just need a gallon or so to get to the next town" and she replied, "Well, that's too bad to be you then, isn't it?" Then turned her back and walked away. Ironically, they took checks (which I also didn't have).
I pack for a long trip the exact same as a 5 minute trip EXCEPT I take into account the potential weather changes. The biggest time waster is rain (or snow) and wet clothes so if there is a potential for rain take a rainsuit and rubber fold up boots. There is nothing worse than hanging out at a laundromat waiting for someone's jeans to dry because they had no rainsuit.
In October we took a 3 week motorcycle trip in Europe staying in hotels and airbnb: Bavaria, Austria, Italy, Switzerland back to Munich. We saw a lot of riders and interesting fashion compared to North America. Almost everyone we saw was wearing a riding suit. Almost no blue jeans/leather jackets. They wear whatever the day calls for and then, at the bike, put on their riding suit. Sometimes from a saddle bag and sometimes from a backpack. at the end of the ride the riding suit goes back in the bag. Makes sense. MotoCentric Barrier One-Piece Suit - Closeout - Motorcycle Superstore
500 -600 miles shouldn't be any more of a big deal on 2 wheels than if it was the family car. No reason to pack a ton of tools and spare parts. If you think your bike might need all those repairs, then junk it and get something reliable. If you are expecting to do that distance in one day, which is doable, you just won't have much time to do much more than just ride, then all you should need is a bottle of water, credit card and a pocket full of cash. As
alluded to, sometimes cash is needed. For longer trips, light packing might be needed. On one of my first long trips, San Diego CA to Alaska, I learned real quick about overpacking. I found myself throwing stuff away and mailing stuff home. With that said, I stay in hotels. Typically most I stay at have laundry facilities. 3 days worth of skivvies an extra pair of jeans and rain gear is all I pack as far as clothing. I buy t-shirts along the way. I carry the tool kit that came with the bike with a few additions; needle nose pliers, mini crescent and side cutters. For full disclosure, that's about all I pack when taking my four wheeled vehicle on long distances also except I have a small variety of flashlights, some cargo straps (I drive a pick-up) and there's an old MRE under the back seat somewhere. That's about it. I don't carry electronics or extra parts.
Hope to hear about your trip. What you took, what you wished you took and what you were glad you brought along.