今は休暇だから本を読んでるし、泳げるようになってるし、リラックスしてる。
This is just my opinion for this particular sentence, but I'd go with the ～たり form here:
今は休暇だから本を読んでたり、泳げるようになってたり、リラックスしてる。
～し lists either actions or qualities, and while it resembles ～たり in that it doesn't specify an order in which the actions took place, it adds a "not only, but also" implication to it:
今年の夏は海に行ったし、山にも行った。 This summer I went to the ocean, and [not only that, I also went] to the mountains.
荷物が多いし、雨も降ってきたし、どこかで休もうか。 We've a lot of luggage, and [not only this, but] it's started to rain, so shall we take a break somewhere?
(Above examples taken from page 198 of 初級を教える人のための日本語文法ハンドブック)
In sentences like the second one above, where multiple ～し constructions are used to provide reasons, as you saw in the other question, the last し can be replaced with から with hardly any change in meaning:
荷物が多いし、雨も降ってきたから、どこかで休もうか。 We've a lot of luggage, and it's started to rain, so shall we take a break somewhere?
～し can also get tacked onto the end of a sentence when that sentence provides justification for an immediately preceding statement:
せっかくパリまで来たんだから、観光でもしよう。天気もいいし。 We've come all the way to Paris, so let's do some sightseeing. The weather's nice, too.
このお店はすごくおいしいですよ。かなり安いですし。 The food here is amazingly good. And it's easy on the wallet, too.
～し can be used in both informal and formal settings (as seen in the second example immediately above, it can follow the polite form).