Honestly, once you figure out how to land successfully on the Mun and return, everything else I've done isn't that much more advanced. I've reached a technical break, though, because I can't return from an Eve landing with the strategies I've been using, and I can't do anything beyond a flyby with Jool or its moons. I may be able to do flybys of Moho, Dres, and Eeloo yet, but nothing more than that.

I suspect the next thing I'll need to do is start working on learning orbital rendezvous. Then I'll just build something large that is capable of landing on Eve, likely with some solid fuel boosters, and work from there. Get it back off of Eve and into orbit, then have a second ship rendezvous and bring it back.

I've also built one Munar rover, but it's not very practical and I think I'll use it as a Minmus rover. I've recently unlocked the huge Mk3 rover wheels, so I might just build a titan rover and roll that thing around Mun. Will probably need my gigantic interplanetary rocket to get it out of the atmosphere though.

Beyond that, I also intend to start working soon on my first space station and a spaceplane. I've almost got everything unlocked that I'll need for either (one more unlock for each and I can get to work). I have the ion engine, the nuclear engine, the aerospike engine, the RAPIER, etc. I just need better wings for the planes. As for space stations, I'm missing a large variety of things for those still, but all the final essentials are in one unlock, so it won't be long. But to build that, I'll need to practice a lot of orbital rendezvous yet.

Eve is near impossible for a return run, the ship needs to be incredibly sophisticated. The Dres landing is easy, not all that different to Duna. I've heard Eeloo isn't too bad, but I haven't actually done it myself. I tried Moho and failed to get off the surface. It's brutal, every stage requires so much more fuel.

Orbital rendezvous is probably my most satisfying achievement in the game so far. I couldn't even imagine doing it at first, it seemed so hard. After spending a few hours getting used to the RCS controls and how to change orbital speeds, I finally managed to do it, and it has been pretty routine ever since.