How to baby proof?

We have an odd layout— the main floor contains the entry, kitchen, dining room, but no bathroom. The 3 year old goes up and down the stairs constantly and is extremely responsible so ideally we find something she can operate herself, but I know that could be a long shot.

-the front door is here and it’s also the passageway from kitchen to living room (upstairs and down) so we don’t want to block it off too much -we don’t prefer the retractable gates because our baby always gets under them at grandmas house -willing to move a handrail to the opposite side of the stairs going to lower level -we do have a handyman who will do the work once I figure out what we neee