I'll be arriving in the morning, and my hotel is in the Marais quarter. Best way to get there?
Going home, my flight out is early morning, and my hotel is Hotel Muguet.
Any help appreciated.

There are several choices. You'll have to decided which mode of transport is "best" for you.

You can take a taxi, which is the easiest, for about 40-50 €. You can take the RER and then the métro (for me personally, that's a grueling experience after an all-night flight). You can take the Air France shuttle bus to Invalides for 11.50 € and then walk or take a taxi to the hotel. Or you can take the RoissyBus to Opéra and do the same.

I always use a shuttle like ParisShuttle. $56 USD for roundtrip. Last time the shuttle was full so I slipped the driver 5 Euro and suggested that maybe I'd get dropped of first and he did.

btw the Marais is a GREAT location! Have fun!

I don't believe you can take an Air France shuttle to Invalides from CDG, only from Orly.

actually, I see know that you are moving hotels for some reason, so you could take the Air France shuttle from CDG to Gare de Lyon upon arrival, which is fairly near the Marais, and then figure it out from there somehow (taxi or metro). Upon departure from Muguet, you would have to get the Air France shuttle at Montparnasse tower rather than Invalides (it's the same bus that comes in and stops at Gare de Lyon first, then Montparnasse tower). That is AF bus line 4.

Last October we flew in CDG and our hotel was in the Marais. We only had 1 small roller and 1 carry bag so the RER was easy to do. Waited maybe 2 minutes for a shuttle to the airport RER station, bought a ticket at the window, got an RER Express Train, transferred at Chatelet to the St. Paul Metro Station and walked over to the hotel. Because we took the RER Express this whole production took only about an hour. Some people shy away from the Chatelet station because it is big, but it is very well marked and we didn't find it to be a problem. If you have a lot of luggage you would probably be better off with a shuttle or taxi.

I always take the RER and then a metro if I need to. But I travel light - one 21" rolling bag (or smaller) and a daypack or tote. It's never been a problem (you always get a seat as the train is starting at the airport and therefore empty, but it can get crowded as you get into Paris if it's rush hour). It doesn't take any longer than a taxi (and sometimes is quicker)and is a heck of a lot cheaper if you are one or two people. When I am traveling in a group of 4 then I often take a taxi cause it's about the same price. An RER ticket (which includes the metro change if there is no RER stop within walking distance of your hotel) costs 7,50€.