I wouldn't recommend anyone starts on career mode, it's just too hard and too constraining for someone who's just starting and doesn't understand the basics.
But when you have some experience launching silly rockets into space, it becomes a really nice challenge to try and achieve the goals from career mode without having free reign over your technology and funds.
I like the science mode suggestion; gating off complex parts lowers the burden of choice, but without the financial pressure it's just "try this and learn to use it" rather than a race to unlock more powerful gear.
Learning efficient maneuvers to transfer orbits and rendezvous was where the fun is for me. My friend would often build ships with specific delta Vs for me to pilot to a particular body.
Also landing safely before landing gear was added was a great challenge.