As well as something to carry the water. You could put a camelback bladder into your ruck if that fits your needs.
MRE's are pretty great for preparedness. You can purchase entire boxes of MRE's from military surplus stores or online. There are alternatives like dried foods and camp stoves, but you will likely be able to acquire food items like that after a few days on the go anyway. MRE's are usable without any additional end-items required and can be stored for decades before use. If you are going to purchase MRE's though, open every MRE bag and field strip all of your MRE's so they are ready to go. That means removing all of the inner boxing and most of the redundant items that come in each pack. You can essentially pull out all of the foil packs with actual food in them within each MRE, recombine and streamline them as you see fit, then bundle your completed meal builds together with a thin band of duct tape. Each of these compact food balls is then much slimmer and lighter than the original MRE, which will allow you to carry more usable calories. (You also probably only need 1 or 2 of the MRE spoons even if you are carrying 10 meals. They are pretty durable spoons.) There are a number of other benefits to carrying MRE's, most notably that they are scientifically formulated to give you a shitload of calories and all the normal vitamins and nutrients you need to actually survive.
Also, don't plan on eating three meals a day. Under survival conditions, you should try to ration yourself to a single meal per day in order to keep functioning for as long as possible while searching for additional food supplies. It will suck, but you will survive just fine. You even get used to it after about a week.
You wont function well if you can't sleep, so you will probably want the following items:
(With the above combo you will never need a tent! Just wrap your ruck in your poncho to protect it from rain, and you ball up in your waterproof 1-man cocoon.)
And you will probably want a weather proof bag to protect all the items in your ruck in case you need to keep moving in the rain.
In the modern era, electricity is important for keeping your gizmos functioning. The longer your gizmos work, the more likely it will be that other people's will not work, thereby placing you in a positional advantage by default. If you are traveling on foot, it may be helpful to bring a passive charging kit in your ruck to keep your gps, nightvision goggles, and any advanced sensors or optics working. I personally have, and would recommend the following:
As a general note, try to streamline all of your devices to use the same battery type like AA, then you can carry a set of rechargeable AA's for the solar panel.
GPS (Go with a water proof stand alone, or put an offline GPS app on your phone like Oruxmaps. Oruxmaps will let you download basemaps to make an offline map)
Editors Note: This section is for more extreme survival situations where other people become a threat. It is not an endorsement.
Edged weapons can prove useful for utility work or fighting, so I would go with at least 1 knife, and maybe a longer weapon as well.
It can be useful to have a utility vest to carry ammo or other items (like your gizmos), SKD tactical has just about anything you could ever want in that regard. You can go from light chest rigs all the way to full fledged plate carriers with armor.
As far as pistols go, I would recommend just about anything from:
I personally own a USP Tactical chambered in .45 ACP, and a P2000 SK chambered in .357 SIG, but they have some newer models out since I bought mine.
H&K pistols go for a premium but they are worth every penny. The have cold hammer-forged polygonally rifled barrels that are all able to withstand double chamber pressure. Basically it means you can have a bullet stuck in the barrel and fire another bullet to clear it. They can really do that.
Additionally, the higher end models typically have a locking o-ring to ensure maximum accuracy as the barrel and slide remain totally locked together until the extraction phase of firing, and they also have threaded barrels for easy attachment of flash and noise suppressors. Almost all of their pistols also sport some kind of tactical rail that will allow for easy attachment if visible/infra-red lasers. If you do purchase a pair of night vision goggles I would HIGHLY recommend purchasing an infra-red laser to go with it. You can only see an infra-red laser beam if you also have a high-end night vision system, which is a huge advantage for night engagements.
One of the most important considerations in purchasing a specific model will be ammunition. For 'go-bag' type scenarios, I recommend purchasing a pistol chambered in a popular military or law enforcement caliber as ammunition will be more plentiful if you need to scavenge for any. 9mm, .40 S&W and .45 ACP are probably the best calibers to choose from.
H&K offers a number of "trigger packs" for all of their pistols. I would recommend NOT getting any of the law enforcement trigger packs. The LE packs are all designed to maintain a mid-weight trigger pull so that law enforcement officers don't accidentally shoot someone when they have their pistols drawn. You SHOULD get a single-action/double-action combo trigger pack. This is typical for military use. It will allow the weapon to be fired with a cocked or un-cocked hammer, with a dramatically reduced pull weight for a cocked hammer. (the typical state of the hammer after the slide cycles during firing)
For the P2000 SK I ordered a V3 trigger pack, which is the SA/DA combo. On that note as well, find a pistol dealer that can order directly from H&K because you will be able to customize every aspect of the pistol before it gets to you. So you can choose from trigger packs not commonly seen on the market. (For example, the P2000 SK typically ships with a LE trigger pack as it is considered a police carry model, but I got the military style SA/DA)
Other than that, it's all about how large a pistol you want to carry and whether you value a greater quantity of ammo or greater kinetic energy per round.