Tips for the hike:
Difficulty: It's a two-hour hike to the top (three for the actual summit) and then about the same amount of time down.
The first hour is fairly easy. The second hour requires navigating over volcanic rubble and doing some light rock scrambling. If you have bad knees or ankles, this is something to be mindful of.
All that said, it's not particularly difficult and I expect most people will have no trouble.
Shoes: I wore hiking boots, but my girlfriend opted for sneakers, as did the guides. If all you have is sneakers, you'll be fine, but the route is slippery and uneven. I was thankful for the boots.
Clothes: The tour company kept telling us that it was going to be freezing during the hike. I wore shorts and a t-shirt and was sweating. Surprisingly, it got chillier when the sun rose and there was some wind, but I was never uncomfortable.
Use your best judgment. If you get cold in mid-60s (16 degrees Celsius), bring a hoody or a light jacket.
Snacks and Water: The tour company provided us with a bottle of water each and a breakfast box of hard-boiled egg, banana, and two slices of bread. I highly recommend bringing an extra water bottle and snacks. Though the hike isn't long, it's takes a fair amount of energy. You'll be happy you brought beef jerky or almonds.
Bathrooms: There's a bathroom at the base of the mountain and none on the mountain (except for nature's toilet, of course). The bathroom at the base is, let's say, a hovercraft situation.
Hiking in the dark: Be careful! Our guide gave us each flashlights, but I was kicking myself that I didn't have a headlamp. It's not particularly easy to scramble over rocks while holding a flashlight.
Warning: Some tourists try to hike the mountain alone and then encounter irate villagers near the mountain who sometimes harass them if they refuse a guide. The tourists have characterized this as "the mountain mafia."
I'd encourage you to view this another way. The villagers are very poor and tourism is increasingly their main source of income. They maintain the mountain trails, ensure that people don't get hurt, and protect the holy sites (there are a number of temples on the mountain).
Don't be a cheapo. Just pay the ~400,000 IDR ($28) for a guide.