Low numbers are very similar in all Slavic languages, but decimal numbers are very different in Russian. Also some days of the week are quite easy to guess like pondělí, úterý, středa or čtvrtek. Although I´ve never studied Russian, I understand a little of it when people speak slowly.
Same with me and Czech, but very little. I do have a Serbian friend, and he messages me in Serbian, and I write in Russian with some Ukrainian words that are more similar to Serbian than Russian is, and we pretty much understand each other, at least the general meaning. I love Slavic languages