WiFi extender/repeaters?
I've had good results with a HomePlug wireless extender - specifically, NetGear's "PowerLINE WiFi PLW-1010 Essentials Edition".  One end - a HomePlug AV2 network adapter - connects to the existing access point via an Ethernet cable and plugs into the house's power, making other adapters on the same[1] household power system appear to Ethernet attached to the access point.  The other end - a HomePlug AV2 wireless adapter - plugs into a power outlet elsewhere in the house and runs as a WiFi access point.  In the specific case of the PLW-1010, both adapters are part of the same package, and the WiFi adapter can auto-configure itself to clone the existing WiFi name (SSID) and password.  Thus you can walk from room to room and bounce between access points seamlessly.

[1] "Same" can get pretty technical, and have lots to do with where electrical transformers are in the grid.  All HomePlug data on the electrical side is encrypted, and any HomePlug device will let you choose a non-default encryption key, for cases where "same" accidentally includes your neighbor's house too.