The words both mean very similar things, so they CAN be used interchangeably without being wrong. Their difference lies in the contexts they are generally used in and with what words they are used. A collocate is a word that typically is placed next to another word. I will provide some collocates so that you can see how they are used. Fabulous means excellent or extraordinary or of or relating to a fable Splendid means excellent or impressive Fabulous: The student did a fabulous job on his test. You look fabulous today. Your new jacket looks fabulous. The man is fabulously wealthy. Splendid doesn't work here We had a fabulous time in Hawaii. There are fabulous creatures in the story. Splendid doesn't work here Splendid The weather is splendid today. We have a splendid view from our hotel. The resort we stayed at was splendid. The meal was splendid and definitely exceeded expectations. That country has splendid beaches.

'The scenery is fabulous' is fine. Though, it's one of those words you have to be a little careful in usage. Only females tend to use it where I am. A male saying it would usually sound effeminate. 'splendid' has your meaning, but is no longer used - sounds very dated and a little snobbish if you used it.