what are the best recommendations

Hi Claudia,, I would repost this on the "Transportation " forum here, but even if you don't we do have a few posters that are really great at suggesting routings etc and look in on this section of the forums too. From Paris to London I will tell you the best option is the Eurostar,, and the best prices for that are available if booked well in advance of travel.. 60-120 days in advance are best. How long is your entire trip?This is a big factor,, time spent travelling should be in proportion to trip length,, if you only have 7 days why visit 5 places?
What others places interest YOU,, ( everyone has opinions based on their own interests,so while there are no wrong answers on what to see,, it is best if you suggest things you are interested in for people to give you recommendations).

Claudia, A bit of further information would help. For example, how long is your trip? Why are you starting in Hannover? What other places are you interested in seeing? It should be easily possible to fit both Paris and London into an Itinerary, but there's not much info to work with so far. Cheers!

Hi Victoria, Thank you for the reply, I am starting in Hannover because my boyfriend is going there for work so I am tagging along, I have never being to Paris and he has never being to London that's why we would like to cover those 2 places but we are on the opposite side to begin with so we feel we are wasting an opportunity to see visit somewhere else, we have 10 days total.

Hi Ken, Please see my respond to Victoria; thank you for all your input...greatly appreciated!

I think you would be very happy to split as much of the the time as you could between Paris and London, without stopping elsewhere. However, Amsterdam is a great town and not far off your route. Beware of seeing so many places that you do justice to none of them.