From lefkas island you can go south to many nice destinations : kefalonia -ithaki- Zakinthos and many smaller island around ..
The most beautiful beaches are around you .
If you have only one week don't wasted going north
North you have the beautiful paxous island and after that Corfu which need more time .
But if you go up north make a stop to prapa mali beach half click south from Karavostasi beach (between Perdika and Parga
Nice underground local people with some nice little (20-30 people )fullmoon beach party's ..
Doesn't matter if there isn't fullmoon , they will be there anyway
Grab some
rum and join them .
39 19.452 N
20 17.616 E
Also called Aquila on
Navionics charts (named wrong but it will help you find it )