I like prowling through the research materials and figuring out how I can replicate a certain part that isn't in the kit or needs to be improved, especially if I'm doing a conversion. For instance, the tornado chase vehicle I'm currently building is not a model of a real one but a hypothetical one. (The reason: Most real tornado chase vehicles are minivans and SUV's, but there aren't many kits available of those two categories of vehicles, so I had to improvise with what's available.) I had fun prowling through the websites of various storm chasers and researchers and checking out the pictures of various storm researcher's vehicles. I'm combining several ideas into my tornado chaser.
Assembly is fun, too. Every part that goes into place is one less part I have to worry about. And there is something satisfying about watching an umpromising pile of parts and pieces slowly becoming a completed model. It's always a red-letter day when you glue the fuselage halves together and say, "It's finally starting to LOOK like an airplane!" Or you install the engine in the chassis and add the four wheels, and it's starting to look like a car!
While we're on the subject of assembly, scratchbuilding your own parts is fun! Because I work so slowly, I do resort to buying aftermarket parts too, but I always try to make room for at least a few scratchbuilt items. A few pieces of sheet, strip or scrap styrene, a hobby knife, some glue, and finally the paint, and you've made a CB radio or a shovel.
I also like painting--not so much the tiny parts but the major parts, like the entire airplane after it's been assembled, or a car body that's ready to put on the chassis assembly as soon as the paint is dry and polished. Once you reach that stage, the model is usually 85, 90, even 95% finished; all you have are a few details.
And then one day, you attach that last part and put the model in the display case. I love the feeling of finishing the model and looking at it under the protective cover.
And then it's time to start the next model! That's fun, too!