Food
Second, bring food. Here, there are some general rules that always apply on the trail, but I think you should also have some fun with it. First, the general rules.
Do not litter. Seriously, people who would litter on the side of an unspoiled mountain are the worst. Bring two sealable bags, one for food, and one for waste. If you pick up a piece of litter and put it in your bag, let me know if you see me in a bar and the next beer is on me. Pack in what you pack out, and leave it better than you found it.
Bring calorie-dense foods. Foods like protein bars and even candy have a lot of calories for very little weight. The same goes for jerky. Like water, calories keep your body and brain running as intended, and thus matter a great deal.
But, bring something you really enjoy for meals. It might not be practical, but a sandwich from my favorite Seattle grocery store was truly excellent in the foothills of Mt. Ranier.
Clothing
I’ll take this one top to bottom. Your boots should at the very least cover your ankle joint to prevent a lot of strains and breaks, and they should be tied tightly. Yes, it’s hot in the summer to do. You will sweat a lot more if you break an ankle in gator territory. To help with this, many brands make awesome merino wool socks that wick moisture.
I always hike in pants: I like climbing pants with an elastic waist and a drawstring for their lightness and so there’s no belt chafing at my waistline. One thing I should have considered and would recommend is packing a bathing suit that can double as shorts if you want to take a swim or need to change out of sweaty or wet pants.
I like to hike in long-sleeve fishing tops just to prevent sunburn and bug bites, both of which are unpleasant as they are avoidable with some careful planning. A bandana and a hat are also good calls for this. The hat, bandana, and shirt can be soaked in a cold, fast-moving stream for some seriously helpful cooling on long hikes.