> 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<b>たり</b>、泳げるようになって<b>たり</b>、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<b>し</b>、山にも行った。 This summer I went to the ocean, and <b>[not only that, I also went]</b> to the mountains.

> 荷物が多い<b>し</b>、雨も降ってきたし、どこかで休もうか。 We've a lot of luggage, and <b>[not only this, but]</b> 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](http://books.google.com/books?id=l-C4H2sBJlEC&lpg=PP1&pg=PP1#v=onepage&q&f=false))

In sentences like the second one above, where multiple ～し constructions are used to provide reasons, as you saw [in the other question](https://japanese.stackexchange.com/questions/474/the-many-ways-to-say-and-in-japanese), 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<b>から</b>、どこかで休もうか。 We've a lot of luggage, and it's started to rain, <b>so</b> 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).

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Mark Hosang kindly pointed out in the comments that in formal speech (and perhaps most commonly in written Japanese), the ～ます stem form of the verb する (し) is used as a transition between clauses, just as the ～て form (して) is used in spoken Japanese. From ALC:

> 上海の不動産価格は去年1年間で24％<b>増加し</b>、今では中国で1番高くなっています。

> Shanghai's real estate prices <b>increased</b> 24 percent over last year, and are now the highest in the country.

This is unrelated to the ～し form discussed in this question.