You can cover:
Paris - 1 or 2 days based on what exactly you want to visit. only eiffel means 1 day is enough.
Switzerland- 4 -5 days for visiting both jungfrauch and mt.titlis .
Venice - 2 days to see the islands near by and venice.
other than this , if you like beaches can opt for costa brava ,islands near spain and islands near greece based on the weather conditions.
If you want to visit only places with 1 day visit, can try: