"Where can I buy a ticket to get around/go sighseeing london by bus"
"Where can I buy a ticket to get around/go sighseeing london on bus"
To 'go sightseeing' means to travel around the city looking at the different interesting places. (You keep missing out the 't' in sightseeing)
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We do not 'sightsee' a place because the verbs 'sightsee'/'go sightseeing' do not take a direct object. Sometimes 'sightseeing' is used as an adjective: 'a sightseeing bus'
< Other question now has its own thread. Cagey, moderator. >