The first question is when will you be taking this long weekend? In the next month or so, before Christmas? If so, you might want to visit a town or city that has a charming Christmas market. Others can make some suggestions, but I would think it would be a city in Germany or Scandinavia.
Paris is always an option, and a trip via Eurostar would be easy. I'm not sure why you have particular concerns about the "safety/ease and manageability/friendliness" of Paris. Are you concerned because you don't speak any French? That's going to be an issue in almost any continental location - except maybe in Germany and Scandinavia where it seems that almost everyone speaks excellent English. But in Paris, even if you don't speak French, you will find during a weekend that a few basic, polite phrases and behaviours will get you far: remember to always greet people formally who are providing services - Bonjour Monsieur/Madame is essential whether you are stepping into a taxi, getting onto a bus or entering a store. And when you leave, remember to catch the person's eye and thank him or her "Merci, monsieur, au revoir". Very simple phrases, but very important.
In terms of danger, I've lived in London, New York and now Paris, and I don't think Paris is any more dangerous than London - less so, probably. There are a few areas where you need to be very careful with your belongings (mostly the very touristy sites, as well as a few metro stops). But I feel much safer here, walking around by myself at night, than I ever did in London or New York.
What do you mean by manageability? Again, is it a question of not knowing the language? You'll still be able to read/decipher maps, metro routes and street signs. A lot of Paris is very walkable, there are some great bus routes that take you past major sites, and the metro is actually quite easy to use. You could also buy tickets for the hop-on-hop-off bus and get around the city that way.
On the other hand, if you have some mobility issues, then you might find Paris to be a bit of a challenge. Like many other European cities, a lot of public places are not very accessible to people with limited mobility.
Some other cities you might consider for a short visit are Copenhagen (smallish, friendly, lots of English speakers, interesting sites but easy enough to see in a weekend), Berlin (lots of history and culture to see), Barcelona (along with Copenhagen, one of my favourite cities - but I feel somewhat less safe in terms of pickpocketing risk etc in Barcelona than I do in Paris), Granada, or Bordeaux or Lyon (if you'd like to visit France but want something a little warmer and a little smaller).