Basically, the idea of my list above is to give lots of ideas that you can have things prepared for, or just "grab and go" with something.
Yes, lots of regular outings or activities can be festived up - look at websites such as Activity Village, DLTK, cBeebies, Crayola, and homeschooling sites, to get free printable colouring pages, activity sheets and ideas for easy crafts (making snowmen, penguins, Santa all out of cardboard loo roll middles, lots of ideas with paper plates, and all sorts of other things to give you inspiration). Many of these will be done in Christmas, winter or other seasonal themes. Homeschool sites can be handy to get a theme and give ideas of how to do a good few different things with it - like making penguins as a craft, colouring sheets with simple activities (shapes, simple addition etc), a writing sheet with penguin pictures, a story involving penguins, an idea for baking/food, what to look out for on a walk ....all linked together around that week's theme (in this case, penguins - although maybe not so much for a seasonal walk in the UK!). And lots of other themes as well.
Woolly's idea of packs prepared ahead of time is good too.
Thinking of things that you can supervise, or be marginally involved in from a chair as you feed newborn (however you feed), or having stories to read while you feed or snuggle with a sleepy/napping newborn and older DC together, would be good.
We used to do a Christmas story for bedtime throughout December, and usually had a regular story as well. Even last year, at age 11, I was still having to do the French book we have of the mouse family's daily preparations (I have to read it in French and then translate to English) which we've been reading since DD was about 7 (she took an afterschool class in French that year and that book has stuck ever since!). There are absolutely loads of great Christmassy books for different ages - anthologies with poems, seasonal extracts from novels and short stories to pick something different every day, Mr Men seasonal books, Thomas the Tank Engine has at least 1, and things like "Father Christmas needs a wee!". We always put those away again after Christmas until 1st December (and we keep our copy of "Twas the night before Christmas" to read on Christmas Eve for the first time each year).
We also have a stash of Christmas movies and cartoons. Well, some have now gone to DCousins as DD is too old for some, but you may find lots in charity shops etc and on reduced tables in DVD shops. From a few short cartoons on one, to a full length movie, these can be great to watch together while you concentrate on feeding newborn, or on any bad weather days you may be confined indoors.
While baby is napping somedays, you could get older DC to make potato stamp paper to wrap their presents in, or cards for Granny and Grandad's etc.
Other days when the weather is good, get out to park or playground while baby is napping, and let nearly 4yo run, explore, point out things to them etc. Can you make a special "shopping trip" one day, early in the day before too busy, to let them choose a present from them for DH, and have a special hot chocolate and a bun stop somewhere nice while you feed DBaby, before heading home again? (That trip should, if at all possible, have no other shopping that needs doing, you're not dragging DCs around for everyone else, it is THEIR trip (ok mostly 4yo not baby).
In terms of where to have the meal, definitely think ahead about logistics, expectations of others and how it would fall to you (and DH) wherever you were.
So would you be expected to do everything if you stayed at home?
Would they expect you to do lots if you went to them?
Is there a good environment/atmosphere with others around (whether at home or elsewhere)?
Do you all like full traditional turkey dinner or something different?
Is the expectation of a formal meal a rigid one, with set timings etc, or is it more relaxed?
Do you have space to cater for everyone?
Would you have to host with beds as well, or just the meal/few hours in the middle of the day, or from early arrivals (including breakfast) until late evening but no sleepers?
Would people bring something if asked, if you host?