Interplanetary has always been one of the attractions of KSP to me. The orbital mechanics give good insights into how it is done in the real world. That has always been a bit of a mystery to me. KSP has been a great way to learn about stuff that would otherwise seem far to hard. KSP allows me to try it out, get it wrong and get away with it Better next time.
My first interplanetary mission was just two days after getting KSP, with a rocket that was designed to look something like a Saturn V, because I figured NASA knew what they were doing, and KSP had parts that looked similar. It worked, and after a frightening mun landing, I set my sights further afield. I'm pretty sure I just did it the simple way, first escape Kerbin, then muck around with nodes to get an intercept. The Saturn V was grossly overpowered. Scott Manley's videos really helped a lot, even though I didn't really understand them all that well at the time, but I did learn that Duna was much easier than Eve
I've landed on every planet and moon since then, many times, got off eve from sea level, and got a good slingshot for a maned Moho landing and return on just my third career mode launch. I love getting a slingshot from Tylo for Jool insertion without burning any fuel. But orbital mechanics always seems to have more to learn. Those Rich Purnell NASA folk do some pretty amazing stuff, something you gain an appreciation for when you try to emulate them!
Still, aside from science and a few easter eggs, there's nothing much to do once you get there. I can finished the career mode tech tree on Ike without visiting any other planet, and could probably do it from just Kerbin, Mun and Minmus, but that's just a grind.
I wish there was more to do on the planets! Like anomalies to be investigated that are unique to each game. Local exploration that makes it worthwhile setting up a base on a planet. Treasure hunts of some kind perhaps. The trail of a long gone alien race or something. It would be cool if specific areas that are identified in contracts, or discovered through other fairly obvious clues, had bonus science. Maybe add "Alien Artifacts" as an additional score, that reveals further interesting locations through contracts.
Setting science rewards lower does help, so you do more missions before science becomes worthless, but perhaps there could be more focus on more interesting contract based bonus science locations rather than just landing at every biome. I recently got 26,000 science from one mission to Duna and Ike before I got bored and went back to Kerbin (testing 1.2). That's the whole tech tree in one mission (although not the first mission!), and there was at least twice that available.
Then there's the maneuver node system is, which is, shall we say, basic. There's a few tricks to learn here to get the maneuver you want, and it can be frustrating at times, but it is amazing what can be done with such a simple system. I've never used a mod to plan a maneuver, other then Precise node that just makes it easier to adjust the nodes.
I've never been that much into space stations or space planes. But the Mun and Minmus are always good for quick missions and minimalist challenges.
Some of us do fly interplanetary!