just to add on a few suggestn
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hi, you can take the river ferry, i think it's 12 baht from pier below sakphan taksin BTS station. then go to tha tien pier. this is good for wat arun, sunset and night scene is good here, you can eat at one of the restaurant here which has 2 levels, which has pretty good view. Or you can take a walk out to the main road and go backwards and you will find another small jetty which i think has a great wat arun view.
another place you can try if u have time is damnoen saduak flaoting market, I took a half day package about 1000bt, quite expensive i think. But this place is worth it. there's a relatively quiet place (less touristy, gotta walk right through the sheltered shopping cross a bridge and walk to another end). This has some an old charm, with traditional shops and the river is not congested.
some other nice bangkok place is the very popular erawan shrine (4 faced buddha), lotsa ppl pray here, nice spot to take pictures, either from the skytrain walkway (need a telephoto) or from inside the temple, day or night, it's just as crowded. it's in between chit lom and siam bts station.