I use a pannier messenger bag. I can carry it over a shoulder to the bike, then it clips onto the rear rack. I got this one: https://banjobrothers.com/products/banjo-brothers-market-pannier

I used it for years until it finally wore out, and then bought a new one because it's so simple. It's just a big, rugged sack. I keep it by the door, and if I need anything (computer, jacket, snacks, change of clothes for my kid) I toss it in, and when I'm getting onto my bike I'll often just toss my pocket items in. If you don't want them floating around too much it has a little pocket on the outside and a little pocket on the inside.

As I said, I really like it's simplicity. It's not fancy, but $70 for a really durable go-anywhere bag that pops on and pops off pretty much any bike rack has really worked out for me.