What to do around Cartagena for 70-year-olds?
 

We will be taking my parents in law to Colombia over NYE.
We'll arrive in MDE on Dec 26, then on Dec 29 we'll go to Providencia and on Jan 3, we'll go to Cartagena.
On Jan 5, my partner and I have to leave, but her parents will stay there until Jan 9 (no earlier award flight back home), when they will fly home via BOG.

While I haven't been to Cartagena before, staying there for 6 nights sounds a bit excessive?
Maybe it would be better to send them somewhere else? They don't speak Spanish and only limited English, though (they are German).
They are about 70 years old, but still very active (they easily out-party me).

We have thought about sending them to the Rosario Islands, but that's probably not that exciting after having been to Providencia?
We could send them to Bogota, but I guess it will be cold there in January.
Any other suggestions?

Three nights is plenty for Cartagena. Two full days is ample. Three days at a stretch

Not only is there not much to do around Cartagena for 70 year olds but save for enjoying the ambience of the city there is not much to do for anyone at all.

They can go along the coast to Santa Marta and stay somewhere with nice beaches for a few nights then fly SMR to BOG to go home. Accommodation is getting tricky to pin down but I happen to know Hotel Aité has a double room with partial sea view from 4-9 January so worth looking at an option like that.

Bogotá has a tropical mountain climate so Bogotá does not care that it is January. It is not going to be cold. I like Bogotá in the first year of the year, fewer people around and less pollution. Normally it is sunny and blue skies but this is not predictable. Last year it was only sunny on Boxing Day and rained like crazy until about May

If your parents are into history then you can maybe consider a side trip to Mompox from Cartagena.

Have a great trip and definitely go to El Baron and Alquímico to get mashed up on fruity cocktails. Also Epoca is a godsend in the blistering heat. It's a tiny chain of two coffee shops that have air conditioning and light lunches. Worth pinning them to your map before you leave as you are definitely going to want somewhere to go to sit down and cool off a bit.

you can go with them to the gold museum, the white beach, or go to a place where they can dance salsa

I kept on popping into the Gold Museum in Cartagena to cool down. Entrance is free and as it consists of several air-conditioned rooms you can visit one room at a time. Try and avoid groups from cruise ships when they are visiting the museum as it’s quite small.

One of the most memorable meals I enjoyed in Colombia was at Restaurante Interno, inside the women’s prison in Cartagena. An early reservation is essential.

Or the Rosario Islands? But it looks like decent accomodation there is already fully booked for January, and/or super expensive...

In Cartagena try to visit its cultural places, like the Clock Tower, Heredia Theater or San Felipe Castle. You can’t miss Baru Island; it is a really beautiful landscape and you can spend the most relaxing day at the beach in Playa Blanca.

Then, if you want to escape from cold weather, choose a place near the beach. For example, Santa Marta or another Caribbean destination, like Panama.

For us, visiting the National Aviary of Colombia was a highlight. It's kind of like a zoo but only features native birds. It's brand new and is about 15 miles from Cartagena, We join a day tour there, which included a rustic beach front lunch at Playa Blanca.

We were in Cartagena for 4 nights. Just soaking us the ambiance was satisfying. Cartagena has nice museums and great restaurants.

We recently returned from six nights in Cartagena. We had a couple pool/beach days in there, but we found plenty to keep us busy. A great trip.

Would you rather stay in the old town or in this new area where the Hyatt is? I heard it can be quite noisy in the old town?

Definitely old town. We stayed in a guest house just outside of the restaurant district and it was plenty quiet. I would probably stay inside the tourist district next time because it's so lovely at night.

The Hyatt is on the beach outside of town. Beach is lousy: grey, rocky, and very narrow at high tide. It isn't Miami Beach.

Hmmm, I booked the Hyatt on points (Jan 3-5) during the recent Hyatt points sale, I don't think it's possible to cancel.
I know that the beach isn't that great, but I read that the Hyatt has very nice pools and isn't too far from the city. So I thought it might be good for relaxing during the day and a good starting point for day trips...? Or was it a complete mistake?
Also my parents in law will probably be staying in Cartagena 4 more nights after that - they could either stay at the Hyatt or I could book them into a hotel in the old town?

I'd stick with the Hyatt, at least for your stay. Old town is a $3, 10-minute taxi ride.

As long as you aren't by a club of the main entrance old town should be quiet.

Eat, eat and eat more. I had a great time out there.