I had mixed results with a WiFi extender - it worked at first, but very quickly broke down and the replacement I purchased didn't work.
I've heard of something called powerline adapters which somehow manage to allow you to run internet connectivity through your electrical outlets, but I've heard generally that you lose a bit in connection speed. Alternatively, I don't know if anyone asked about how old your current router is - generally speaking the advance of technology improves range and signal strength, so if it's more than 2 - 3 years old you might get a boost just by replacing it.
Of course none of it, like others have said, would really be a substitute for just running ethernet properly - which might be the most cost-effective option as well.