Når jeg våkner etter en lang drøm, kan jeg fortsatt lukte kaffe på kjøkkenet.

• når is used for general, habitual or future events and for asking about time (e.g. “Når begynner timen?” – “When does class start?”).
• da refers to a specific, completed moment in the past (e.g. “Da jeg våknet i morges, var det snø” – “When I woke up this morning, it was snowing”).
  In your sentence, når signals “whenever/when I wake up” rather than one single past event.

Using the simple present (våkner) makes the statement general or immediate: “Whenever I wake up after a long dream…”. If you wanted to describe one past occasion, you’d switch to past tense with da:
“Da jeg våknet etter en lang drøm, luktet jeg fortsatt kaffe…”

• en lang drøm refers to “any long dream” (indefinite singular common gender).
• kjøkkenet (“the kitchen”) is definite because you’re referring to a specific place in your home.

In Bokmål, drøm is a common-gender noun in the indefinite singular, so the adjective stays in its basic form: en lang drøm.
If it were a neuter noun (et), you’d add -t (e.g. et stort hus). If it were plural, you’d add -e (e.g. lange drømmer).

Lukte (to smell) is transitive when you smell something: you smell coffee (lukte kaffe).
If you want “to smell of/like something,” you’d use lukte av: “Rommet lukter av maling” (“The room smells of paint”).