Accommodation: Basically, most hotels recommended in lonely planet is OK. But, please check the address and make sure you are brought to the correct hotel. All hotels and shops have their address written on the sign board. Hotels that are highly recommended in guide books are easy targets of scams.
Transport: You can either take a cab from the airport to your hotel or take a bus to old quarter. Always ask to use the meter if you take a cab. I personally like the cab which are white and with a green top. The drivers wear a white shirt with a green tie. These cab drivers speak good English. If you get into a cab, and you notice that the meter jumps super fast, you are in some kind of trouble. What i suggest is, come up with some excuses like you need to go to the washroom and get the cab to stop. pay whatever is on the meter and leave. If i'm not wrong, there are also private buses which bring you from the airport to the old quarter as well.
Food: I cannot really comment on this. This is kind of personal preference. I do not really like the beef noodles or whatever noodles that the locals eat for almost every meal. But, that's just me.
Money: It is good to have some USD with you. But generally, I would still think its better to work with Viet Dong. You can change Dong at the Airport. USD would be good for paying things like the hotel etc.
Navigation: If you really have difficulties doing this, take a cab. Its not really expensive compare to Singapore. But, walking around with a map is easy. All shops have their addresses written on their signboards. So by just looking at the signboard, you will know which street it is.
Safety: Dont worry too much.
People: The locals there are really friendly. Spend some time interacting with them (or maybe I'm the only weirdo who goes around talking to random strangers). But be prepared that most people at Hanoi do not speak english.
Places to visit:
1.) Sapa. I recommend you to do a homestay. But be prepared that the living condition is nothing great compare to your hotel. For me, It was'nt too much of a photography trip. I spent more time talking to the people there than to take photos. But the scenery there is lovely. Bring a sweater with you. If it ever rains, it's going to be cold. You can either book a Sapa tour from your hotel in Hanoi, or buy your own train tickets to Sapa and book a tour there(I think you cannot do home stay without a tour guide). It is definitely cheaper to book a tour in Sapa but i need to warn you that getting a train ticket during peak season is not easy. Tour operators usually buy in bulk (Thats what they told me).
2.) Halong Bay. Some people think its too touristy. But not for me. Spend at least 2 days here. Do not go for the 1 day tour. The journey from Hanoi to Halong Bay takes like 3-4 hours IIRC. So basically the 1 day tour you will only spend a couple of hours there. For me i stay 1 night on the boat. Other backpackers i met along the way told me the resorts along the beach is really nice. But that is for the 3 days tour.
3.) Water Puppet. Watch the water puppet show. Its really interesting.
4.) I'm not sure if you will like this, but i do think that Dong Xuan market in the old quarter is worth a visit.
5.) Others. Of cos, there are lots of Pagodas and Museums you can visit in Hanoi. I think you can easily find information on the Internet so i will leave you to decide where you want to go.