Emill, a few other suggestions for "easily accessible" Medtner works would be op 9 #2, op 14 #2 (search for andhow04's recording of it on this site), or op 38 #1. I agree with David that the works can be challenging to understand, and I certainly can't hum most of them (something which for me is often a surrogate marker for accessibility), but these three selections should whet your whistle for the harder works. I'm a big fan of Medtner overall, but I do agree that it takes a certain investment on the part of the listener to really digest the pieces. They're all very densely composed and most are relatively sparse on melody, instead focusing more on counterpoint and texture. structurally, he wrote a number of sonatas, but he also made liberal use of a form he called the "skazka," which is russian for "fairy tale;" most of which are relatively short and simpler in construction than the sonatas (although this is a generalization that does not hold true for all of them). Also, if you are interested in concerti, he wrote 3 for piano, of which I most prefer the 2nd, although they all have merits.
happy listening!