my advice: 5 days is WAY too short for europe...
so you'll have to choose either you want to see lots of details in like one or two cities or see the basic touristy stuff that doesn't take a lot of time which would mean you could see like 4 cities...
either way you'll be seeing so much it'll be impossible to take it all in.
If I were you I'd just stick with France... Spend a few days in Paris and a few in Brussels... Paris is very expensive though so staying there will cost a lot...
Don't leave ANYTHING important in a hostel if you've booked for more then one night!
talk to other tourists in the hostels... you'll meet the most interesting people there and they will be able to tell you lots about what they have done and what they liked etc. besides that if you make a few friends it will make it a lot more fun then just wandering around alone.
don't go into the louvre unless you plan on spending your entire trip there... it's too big to handle in one day and there are more interesting things in Paris like the eiffel tower.