This is a question for the users who write music in a DAW; what is your process for starting a new song? Do you have an idea of what the song sounds like and go from there? Or do you mess around with instruments and keys until you find something that sounds cool? Which methodology is better?
In addition to using the DAW to mimic traditional songwriting steps -- doodling on a VST piano, or jamming with drum loop and arpeggiated bass -- you could use the DAW's features to seed the creative process. How about
- try the different preset sounds (as you suggested) and try them with different effects too: a flanged piano will lead to you playing different rhythms and cadences than for an untreated instrument; a blippy echo on a synth may suggest a different tune.
- try and recreate the structure and chord of a published song (can be good practice in refining your technical DAW skills) but make a conscious effort to diverge, dilute and replace the original over time.
- Use the DAWs MIDI functions, e.g. Cubase allows you to "delete black keys", or reverse the order of notes.
- record MIDI based tracks using traditional drum kit sounds, then play them back on tuned percussion.
- Try capturing tunes/chords using the list editor or score editor modes (rather than input from keyboard)
As has been commented, there's no best method. Personally, I generally start with piano (not in the DAW, so that I don't get tempted to play with the myriad features), and prefer to have lyrics in mind too. Hopefully, your DAW may help if the Muse is playing hard to get.
I don't know anyone who actually writes music in a DAW. DAW's are good for moving your basic tune to a more finished piece.
Typically you'll get your concept from strumming a guitar, playing on a piano, singing a melody etc. or just jamming with the band.
A simple next step is to pop a basic click track into a DAW and lay down a version, which you can then return to to change tones, instruments, keys, effects etc and add complete drum tracks.
Then, from the producers I know, you have a massive number of improvement iterations, tweaking filters and delays, moving attack points etc., before even thinking about mastering, but I think most of those latter stages do not count as writing a song...
There is no single right answer for this - writing music, using a DAW or otherwise, is an open-ended process and anything that works for you is legitimate.
I personally use both approaches - starting with an idea, and starting with nothing and seeing what comes up - and have found success with both. For my purposes, the second one seems to be more effective, but this is probably because I tend to write electronic music where instrument timbre is very important - I'm more focused on the sound of the instruments than their arrangement, so beginning with an interesting sound I've created in my DAW is a natural starting point for inspiration.