How long before I can walk on fresh concrete without messing it up?

24 hours is fine for foot traffic, just take it easy. I walked on my shed slab the next day and it was solid, no marks at all. The thing to remember is your weight spread out over two boots is nothing compared to what that slab will handle later. Skip the wheelbarrow loaded with tools for a few more days though. Give it a full week before you roll anything heavy across it, and two weeks before you park anything with an axle on it. If the crew put a curing compound on it or it's got that damp sheen still, you've got even more protection than you think. Boot prints happen in the first 4 to 8 hours, not the next day. What you might see at 24 hours is scuffing if you drag your feet or pivot hard on a boot, so walk flat and lift your feet. Cool gray weather is acutally your friend here. Concrete gains strength from a chemical reaction that needs moisture, so damp and mild means it's curing slower but curing better. The nightmare scenario is hot sun and wind pulling the water out of it in the first day. What you don't want is a hard freeze in the first week, that's the only weather that can genuinely mess things up at this point. If nights are dropping near freezing, throw a tarp or some straw over it.

On sealer, you've got time. Most sealers need the slab to be mostly dry inside, and alot of guys wait a full month before sealing a new pour because there's still moisture working its way out. Check whatever product you're planning to use, some say 28 days, some are fine sooner. Sealing too early can trap moisture and cause that whitish cloudy look, so err on the side of waiting. One more thing, if the finisher didn't cut any control joints in it, expect a crack or two somewhere eventually. That's just concrete doing what concrete does, not a sign anybody screwed up.

One thing I'd add since it's been dry-ish where I am: keep it damp if you can. I hit my patio pour with the hose a couple times a day for the first week and it made a noticable difference, way fewer little spider cracks than my neighbor's slab that just sat there. A wet burlap or some plastic sheeting works too if you don't feel like babysitting it. And yeah the gray weather's helping you out but don't count on it alone.

One thing I learned the hard way on my garage apron: leave the forms on for at least 3 days. I pulled mine at 2 and chipped a chunk off the edge with the shovel, and that corner has bugged me ever since. The edges cure way slower than the middle since they're exposed on both sides. Also if you've got a dog, keep it off for a few days, my neighbor's lab left prints in a fresh patio and there's no fixing that.

When I did my backyard slab I kept a couple of scrap 2x10s by the edge and laid them down as a path whenever I needed to cross with gear. Spread the load out, no scuffs, and it saved me doing that tiptoe shuffle every time. Cheap and it worked way better than I expected. Also don't sweat the color being a little uneven or darker in spots right now, mine looked blotchy for the first couple weeks and evened out once it fully dried. It's wierd how much it changes between day one and day thirty.

Update: the 2x10 boards across the slab idea worked great, hauled my stuff over the other side without leaving a mark. Still got the hose out a few times since it's been dry here. My remaining worry is the sealer, is there a window where it needs to go on or can I wait a few weeks without hurting anything?