I am operating with an MLC backpack both on plane and at conference and if it's slightly larger than the locals daypack -- oh well. It would work less well with a rolling carry on of course. eBags Mother Lode Convertible (I used this before), Tortuga, Patagonia etc etc the list is endless of these backpacks. (I am experimenting with a Magma Control-Backpack XL currently but unless you also want to carry on a 19.5" monitor, that's probably not the best choice.)
Another solution is to put the 240gr Bach Itsy Bitsy backpack (which I also have) in the carry on (or checked in). The backpack is big enough: even my T420 laptop+sleeve fit in there and provides structure. Your laptop is likely thinner and lighter. Unlike most such collapsible backpacks this is strategically reinforced where necessary so while it's very low weight you can carry like 4-5 kgs in it without a problem. Probably even more but I didn't dare to test further.
Finally, you can just carry your normal laptop backpack (my choice is the Briggs & Riley Verb Activate but many are good choices, this just happens to be the best :P ) and a small duffel (there's no competition here, the Red Oxx XS Aviator is where it is) as a personal item and check in everything else in a suitcase. I did this before as well.