Big Bus Tour & Eurostar

Take the train it is less of a hassle than flying unless you are on a budget and see a direct flight deal significantly cheaper

For Paris I found the bus tours an enjoyable and beneficial way of site seeing.
At least for us with a toddler it worked well since she enjoyed the open air top deck and often protested more walking.

We actually went with the combo multiday ticket from Batobus that combones the L'Open Tour Bus with the Batobus boats.
That gave us more options and varied routes. Where we stayed was close to both a bus stop and a boat stop.
http://www.batobus.com/en/paris-a-la-carte-pass.html

L'Open Tour and Big Bus Tour are competing companies that offer a similar service, I am not sure one is better than the other.

I was happy we did that, the downside to both the boat and the bus is that they only go one way, you have to really read the map, understand the route of travel and plan your stops in advance or will waste hours riding in pointless circles for what would be a short walk otherwise.
The other big downside is they stop running EARLY so you cannot use them for dinner time. Some don't realize this and wait at bus stops for a bus that never comes and then complain online about said company.

If not with a toddler still good for an introduction to the city but overall not the most efficient methods of travel.

Eurostar is the only practical way of getting between central Paris and central London. Faster than flying, a lot more comfortable and if you book far enough in advance relatively cheap.
It takes about 2h20 and there is roughly one departure per hour.

See here for lots of info and a video: https://www.seat61.com/London-to-Paris-by-train.htm

thank you guys for the great and useful information. I'm excited and nervous at the same time!! LOL

L'OpenTour has 3 routes. Here they are:
http://www.paris.opentour.com/en/lines/

Eurostar is taken from Gare du Nord. Print your ticket from home. Take a taxi from your hotel.
Be sure to give yourself 60 or so minutes - depending upon where you are staying - to get in line, and bring a pen, because you have to fill out the declaration slip. Then you go upstairs to the Lounge until your train is called. There is food and drink and an ATM under the escalator.
Eurostar is exactly like the airport. You go through Passport Control, not Customs.
Not necessary to go 1st Class, 2nd Class is fine. You're only there for 2h20 minutes.

We took the Eurostar to Paris from London and back this summer and loved it. We booked our tickets about 3 months ahead of time and got the best rates. If you do decide to go that route, your ticket will say to be at the station 30 minutes prior to departure... give yourself at least 1.5 hours prior to departure.

With the Rick Steves podcasts & travel books along with other travel books like Fodors, we would NEVER go on a bus tour whether it were big or small. Rick's information and recommendations in his publications are FANTASTIC and make bus tours unnecessary.

You can't embrace the back-door culture of a city that Rick Steves writes about if you're surrounded by tourists on a bus. If anything, you're alienating yourselves.

As for Eurostar, we've always wanted to do that. I'd say go for the train if the price is comparable to flying. You will probably save time going by train as you won't have to arrive at Gare du Nord 3 hours early as you would if you had flown from ORY or CDG. Plus you'll arrive at St Pancras which is Central London.

Have fun!

I think the key to using the bus well is getting a lay of the land on your first day.
You read all of the guidebooks you want, without having been there you won't have a sense of what is close to what, where you are in relation, accessibility of sites, etc...
Taking a loop around on the bus, even if you never get off gives you that plus walking you can only cover so much in Paris for certain sites you need to work in some other form of transit like subway / bus / taxi or boat.
In fact if you were to get off at each stop and look around and then wait and re-board the next bus, that would be incredibly inefficient since you would be waiting around way too much.

Paris is a really big city and has very few pedestrian only zones so the busses have more value there than in some other cities.

After your initial introduction to the city via bus you can then take off on foot with a guidebook and feel much more comfortable exploring, getting that authentic backdoor feel without getting lost.

And then there are those with mobility issues or just toddlers in tow where the site seeing buses offer some extra benefits.

I would take the HOHO bus, if the OP wants to take it.
Depending on the weather - in the summer, the top will be open, and if it's winter, it may rain or snow.
Bundle up.