noun
1
BAD : After lunch we went for sightseeing.
GOOD : After lunch we went sightseeing.
BAD : They wanted me to take them for sightseeing.
GOOD : They wanted me to take them sightseeing.
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go sightseeing, take sb sightseeing, etc (WITHOUT for ): 'Today we're just relaxing by the pool and tomorrow we're going sightseeing.'
2
BAD : We're going to do a sightseeing tomorrow.
GOOD : We're going (to do some) sightseeing tomorrow.
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Sightseeing is an uncountable noun: 'Some people aren't interested in sightseeing.'
3
BAD : We visited all the famous sightseeing places.
GOOD : We saw all the famous sights.
BAD : Nagasaki is famous for its sightseeing spots.
GOOD : Nagasaki is famous for its tourist attractions.
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To refer to places that tourists like to visit, use sights (plural) or tourist attraction/spot (NOT sightseeing place/spot ): 'I've always wanted to see the sights of London.'