Learn Hardcore Japanese: kondo no syuumatu, imouto to kinzyo no kouen wo sanposuru tumori da. - This coming weekend, I plan to take a walk in the neighborhood park with my younger sister.

今度 can mean this time, next time, or even the last time, depending on context. In everyday speech, 今度の週末 usually means the upcoming weekend. If you want to be unambiguous:

この週末 = this weekend
今週末 = this weekend
来週末 = next weekend

Why is の used in 今度の週末 and 近所の公園?

The particle の links nouns to make modifiers. 今度 and 近所 are nouns; with の they modify the following noun:

今度の週末 = the weekend this time/coming up
近所の公園 = the neighborhood park

Why is 公園 marked with を instead of で after 散歩する?

With motion/walking verbs, を can mark the route/area traversed:

公園を散歩する = stroll through the park (covering it as a path) You’ll also hear 公園で散歩する, which focuses on location (taking a walk in the park). Both are common; を highlights the space you move through.

Is 散歩する transitive? What is that を doing then?

散歩する itself doesn’t take a direct object; here を is the route-marker seen with motion verbs (道を歩く, 川を泳ぐ). Note you can also say 散歩をする (“to take a walk”), where を marks the verbal noun 散歩 rather than a place.

What does the と after 妹 mean? Do I need 一緒に?

This と is the “with”/comitative と: 妹と散歩する = take a walk with my younger sister. 一緒に is optional emphasis:

妹と散歩する
妹と一緒に散歩する (a bit more explicitly “together with”)

Does 妹 just mean “sister”? What about older sisters or other people’s sisters?

妹 specifically means “younger sister.” Older sister is 姉. To refer to someone else’s sister politely, add さん:

my younger sister: 妹
someone else’s younger sister: 妹さん
my older sister: 姉
someone else’s older sister: お姉さん You can add 私の when clarity is needed: 私の妹.

Why is the verb in present form (散歩する) if this is a future plan?

Japanese uses the non-past (present) form for both present and future. The time phrase (今度の週末) supplies the future sense.

What’s the nuance of つもりだ? Could I use 予定だ instead?

つもりだ = speaker’s intention/will (“I intend to …”).
予定だ = a set plan/schedule (“It’s planned/scheduled that …”). For your own casual plan either works; 予定 sounds a bit more fixed/objective. For someone else, 予定だ is safer; つもりだ about others implies you’re inferring their intention.

How do I make this polite?

Use です instead of だ and keep the verb in plain form before つもり:

今度の週末、妹と近所の公園を散歩するつもりです。 Don’t say 散歩しますつもりです (wrong). Pattern: plain verb + つもりです.

How do I say I don’t intend to?

Negate the verb or つもり:

散歩しないつもりだ = I intend not to take a walk.
散歩するつもりはない = I have no intention of taking a walk. Past intention: 散歩するつもりだった = I intended to take a walk.

散歩する vs 散歩に行く — what’s the difference?

散歩する = do the act of walking/strolling.
散歩に行く = go (somewhere) to take a walk; emphasizes the “going.” With a place:
公園を散歩する / 公園で散歩する
公園へ散歩に行く (or 公園に散歩に行く)

Can I reorder parts of the sentence?

Yes, as long as particles stay attached:

今度の週末は、妹と近所の公園を散歩するつもりだ。
妹と、今度の週末、近所の公園を散歩するつもりだ。 Word order is flexible; choose what you want to emphasize (topic は is common).

近所の公園 vs 近くの公園 — any difference?

近所の公園: a park in the same neighborhood/community.
近くの公園: a park that’s nearby (focus on physical proximity). Often they overlap, but 近所 sounds more “in my area,” 近く more “nearby.”

Is 今度の週末 ever confusing? What should I say to be crystal clear?

It can be, especially in business or when dates matter. Use:

今週末 (this weekend)
来週末 (next weekend)
再来週末 (the weekend after next) Or give the date.

How do I read the key words?

今度: こんど
週末: しゅうまつ
妹: いもうと
近所: きんじょ
公園: こうえん
散歩する: さんぽする
つもり: つもり
だ/です: だ/です