Opposition for the planets is all listed here in IIS under the "Resources" banner in the left margin:
http://www.iceinspace.com.au/planetopp.html
Not only is the opposition time noted, but the size of the planets and their best and worst opposition sizes and times.
A six inch scope will show a huge amount of detail on Mars. Don't think that big aperture is all that's needed with the planets. While it helps to an extent, the atmosphere is the ultimate determining factor. A six inch will show a much detail as an 8" on all but the very, very, winning-lotto like-of best of conditions. My 5" has shown me not only the polar caps, also the dark green surface details that are constant through our scopes. I've also been able to identify a huge dust storm that wiped out most of the features visible on the planet for several months - at first I thought my scope was a poor sample, then I read what was actually happening on the planet at the time and it all made sense, and once the dust settled, things changed back to how they normally are. The polar caps also have a dark green halo around them. You will also notice the polar caps increase and decrease in size respectively over the course of time with the planet's seasons.
Don't underestimate the capability of your gear. Your greatest asset is your experience with the gear you have. Be patient, as things through a scope take time for our minds to interpret, and many, many details escape our identifying because we rush and expect space probe photographic images. Filters
can help, BUT, but, what these will reveal are not immediately evident, nor do they always appear as we would expect them too. There's no mistake with the small font I used with the word 'can'.
I've mentioned filters (oh man, what I can of worms I've opened!
). Rather than say what filter is best, have a read through this excellent filter guide. It not only mentions colour filters, but also the various nebula filters there are and what they are all best for. With the colour filters, all I will suggest is get just two or three, no more! You really won't use them very much at all. Take your time going over the filter guide, and choose two or three filters that will best cover things for your liking. And remember, what filters reveal won't be immediately evident or show up how we would expect.
Patience, atmospheric conditions, and time will be your best friend with the planets and DSO's no matter how big or small the aperture of your scope.