That is something I struggle with as well.

In the past I've been told that if you stay indoors a lot than getting out into the sunlight can help. There's benefit even just having an open window with the blinds open to let the light in.

Exercise also stimulates the brain's reward centers, so if you can get yourself to start an exercise activity it could wind up being enjoyable by the end. I sometimes find that Yoga helps in this way, where by the end of the 30 minutes I'm regretting that it's over because I'm liking it.