The verb “Быть” (to be, is, was) has no present tense. However when it is used in the past and future tense with nouns or adjectives, the following noun or adjective is usually in the instrumental case. (Example: Я врач. Я был врачом - I was a doctor. Она такая красивая! Она была красивой когда-то. Nominative case is an old form, so you can see both, especially if you read Pushkin, Tolstoy etc.

You can use the both words (actually, that's just the same word in different cases), but if you say "прогулка была приятной" you use more literary correct language

Hello! I think it is because "приятной" is a past tense (прошедшее время). Можно употребить и то слово и это слово. Все будет правильно.

U can say "Прогулка была очень приятная" Explanes: "Прогулка" - женский род (she is mine(not he) - woman type) also it is past(word "была" help us to rdetect it) so u shuld use "Приятная" in past form - and it is "приятной" but in this example word "приятная" also have same sens. In this case doesnt metter what word u use.