Student or Learner
1) Is it correct to say "When I went there, I saw many things.There are lots of trees, and there are many flowers."
2) "I went there. There are lots of things. There are balloons and toys.
"When I went there, I saw many things. There are lots of trees, and there are many flowers."
This would be acceptable, however, while we might be able to assume that the place that was visited still has lots of trees and many flowers, we do not know this for sure. We can only know that at the time of the visit, these things were true. For that reason, we should probably express the second sentence in the past tense, as well.
"When I went there, I saw many things. There were lots of trees and there were lots of flowers."