I'm going to use local names since idk the translation for these things. You can copy paste into search engine and machine translate blogs for food places.

General tip: don't bother too much about Michelin stars in Hanoi, especially in phố cổ, as all the restaurants in there all taste more or less the same. Otherwise you have to queue very long with other tourists. As always, eat where the locals eat.

Hanoi

(make sure to keep masks with you for walking along roads)

Electric buses available here.

Go see uncle Ho, I missed him last time I went to Hanoi. The mausoleum closes early (like 10:30) so you have to get up relatively early for that.

Go around the back of the mausoleum to see chùa một cột.

There are several national museums dotted around the centre, if you have time. Monday is a bad day for museums as most of them will close.

Massive government buildings/military areas in the centre as well. Lots of hammer and sickles everywhere.

Go to Lenin park to see a cool Lenin statue.

On Trúc Bạch lake, there is a cool war monument of John McCain being shot down and kept as POW. His plane landed there. Some USians actually pay respect at the monument even if it's anti US lol.

Go to phố cổ, hồ gươm, văn miếu quốc tử giám.

Eat bún chả, phở hà nội, cơm rang, chè, bún gan, bánh cốm, xôi cốm, bún riêu, bánh cuốn. Honestly there's so much food in Hanoi, explore around.

Ho Chi Minh

You will notice the difference from Hanoi. More lights, more people out, busier and more urban.

Museum by dinh độc lập.

Many parks in general where young people hang out, dance, play sports. If you skate, phố đi bộ is a good spot, even when busy.

There's a big zoo, with a lot of greenery. Zoos are not my thing in general but if you're bored and want to see cool birds and reptiles (they've got a lot of them funny birds at the moment to get them out of critical endangered status).

Several smaller museums by the zoo. You might spot a few US planes that were shot down on display.

There's a city hall and opera house, they look very fr*nch and the latter looks bourgeoise as fuck.

I like walking around phố đi bộ and the city centre area to see what people are up to. Bùi viện is like a party street for tourists and I hate it.

There's Saigon waterbus that takes you around the city via the river. It's really nice.

Outside of the centre, there's chợ lớn, which is a 'chinatown'. Lots of pagodas and chinese cuisine.

There is so much food in general, both local and asian in general, you'd be overwhelmed. Worth trying: cơm tấm, bánh xèo, hủ tiếu nam vang, chả cá lã vọng, bánh canh, bún bò huế, gỏi cuốn, bánh pía.

In both places: If you drink coffee, please try out the cà phê phin there. It's good black, with milk, hot, or cold. There are hundreds of cafes and most stay opened til late. Sit by a balcony and watch the streets and the people. The culture is very relaxed and laid back.

In both places, you can get around one area to the other via motorbike, and if you can't ride one, there are many cheap uber-like taxi services for both types of vehicles. You gotta get the apps for that, a bit of a pain, but everyone switches between them to get good deals. Cycling might be an option but just be careful.