HI Peter,

'J'ai vu des endroits magnifiques' is correct but without context difficult to justify.

You would say :

"Je ne connais pas d'autres endroits aussi magnifiques"

because of the negative as explained in the following lesson -

https://kwiziq.learnfrenchwithalexa.com/revision/grammar/the-partitive-article-in-negative-sentences

Hope this helps!