You'll have lots of replies, I should think - I know I did, when dd2 had her op at 26 months 
Main points: take spare tops for both of you, wet wipes etc in case of post anaesthetic sickness (we had none) or hot hospital sweatiness (plenty). Take snacks and drinks and straws. I sent dh out for ice lollies straight after dd woke up, as she wouldn't drink - ice lollies saved us for the next week.
After the op, they like them to have 'rough' foods like toast and crisps - amazingly, dd found them easier than 'sticky' foods like mash or pasta. But don't worry if she doesn't eat for a week or so: if it hurts, she won't want to but she will bounce back as soon as she starts to feel better.
Some kids have a lot of pain (mine did), some almost none, most are inbetween. But in any case, my biggest tip is to keep on top of pain meds after the op and back home for at least a week. Don't wait for the 4 hours to be up, give meds 5 minutes early - we alternated paracetamol and voltarol suppositores, as dd wouldn't swallow the nurofen or calpol. If swallowing is a problem (easier to explain with older kids, mine was very teeny), ask for suppositories - added advantage was I could (sorry, but it was very easy!) slip one in in the middle of the night without waking dd, to keep the pain control going.
I did set my alarm to give pain meds in middle of the night, in dd's case, but probably wouldn't have done if she'd been in less pain.
In terms of hospital itself: going under anaesthesia was easy for us, because dd fell asleep (having had a tantrum because she was thirsty and I couldn't give her milk) and I carried her in to theatre, where they waved gas under her nose till she was properly under then took her off me. That part is hard - the waiting - so if you can, have someone with you to distract you with coffee for the short time it takes!
dd cried hysterically on waking, which upset me but she didn't remember at all half an hour later - the docs said it was a reaction to the anaesthetic, but I think it was also waking up with strange people around her and in pain. Either way, she calmed quickly once cuddled and was totally unaware of it later. Some kids feel a bit sick when they wake up, but dd didn't at all - and ate an hour later.
One thing they didn't tell me was that small kids can react to surgery with a slightly raised temperature the next day - dd stayed overnight in hospital, then was feverish the following evening. If it goes over 38 or she seems unwell, get back to hospital in case of infection (or call the ward). But if its very slightly up, could be nothing as in our case 
The other thing to know is that the pain gets better over the first few days, then gets worse again around days 5-7 (approx). This is because of scabs forming and falling off, so don't stop the pain meds until you're past that stage.
That's it, really - except the best bit: dd's life changed hugely thanks to her op, and so did ours. She lost the starved spider look and the dark rings under her eyes, slept well (she had apnoea before), and within a month was growing and thriving like never before. I have never, ever regretted the operation.
Sorry for the essay - hope it goes smoothly and don't hesitate to post for hand holding if needed!