I also suggest FFRSS (I wrote it). In one of the guides I made a panorama of Earth-Mars-Earth free returns, it was in the pre-RSS v12.0 version of FFRSS so it will be a bit different now if you are using the latest RSS (which is what the latest version of FF is for). Here it is:
I also did a piloted mission to Mars using a flyby of Venus to get there, the path I chose could have aborted the Mars landing and done a free return to Earth, so you can flyby Venus and Mars and get back to Earth with only a few small mid-course corrections. It's here. Watch out though, the arrival speed at Earth can be pretty high if you're not careful.
Some time ago I searched for a variety of departure times for going from Kerbin to Duna, the premise was that it takes time to prep a rocket and launch pad for a mission so one would want to launch whenever the pad becomes available throughout an entire synodic period. By using different routes I found relatively low-dv launch windows all over the place, though the travel times vary quite a bit. Here's a chart showing the best paths I found during the first 226 days of the game. (Note: 24-hour Earth days, not 6-hour Kerbin days!)
The Kerbin-Duna synodic period is about 227 Earth days, but if you are using Eve to get to Duna then there is a quasi-synodic Kerbin-Duna-Eve period of about 681 Earth days, so this chart should roughly repeat that often. More precise is to check the windows for the specific time you are interested in. Note that I use my Flyby Finder to find the flyby windows, but Alex Mun's porkchop plotter to find the multi-revolution Kerbin-Duna flights (where your ship goes more than once around the sun during the voyage). The multi-rev paths have long flight times though, I only use them for cargo missions.
Every one of these flight types can be run in 'reverse' to get back to Kerbin from Duna with departure times all over the place. Starting with Duna-Eve is a big dv hit though. As for Eve, I've never looked, I would imagine K-D-E would add some windows.