Aharon
דברים יא, ל: הֲלֹא-הֵמָּה בְּעֵבֶר הַיַּרְדֵּן Are they on the other side of the Yarden?
Baal HaTurim notes that the final letters of these four words spell AHaRoN. This is an allusion to the loss of Aharon. As long as he was still alive bnei Yisroel did not need specific directions, as the Clouds of Glory, which were present in the merit of Aharon, were their GPS. Now that he was gone then bnei Yisroel needed detailed directions.
The Chariot
דברים יב, יג: וְתַחַת כִּי אָהַב אֶת-אֲבֹתֶיךָ
Baal HaTurim points out that the word "ve'sachas" also appears in Mishpatim (24:10) "ve'sachas raglav ... (and under His feet)", with reference to the Throne of Hakadosh-Baruch-Hu (also known as 'the Chariot').
This teaches us, he explains, that the Avos are synonymous with the Chariot - inasmuch as they uphold G-d's Glory in the world, even today, by virtue of the legacy that they left behind.
Double Peh
דברים טו, ח: כִּי-פָתֹחַ תִּפְתַּח אֶת-יָדְךָ לוֹ
"You shall surely open (posach tiftach) your hand to him ..." (15:8).
The double 'Pey' denotes that one should open, not only one's hand, says the Baal haTurim, but also one's mouth, to offer him a kind word.
Alternatively, if the poor man needs bread, give him bread, if he needs sweets, then give him sweets - open your hand to him in every way that assists him.
Alternatively, if he is embarrassed to come to you, you bring it to him.
Alternatively, the Torah writes a double expression in all matters that concern Tzedakah - "Posach tiftach", "Nasan titein", "Ha'anek Ta'anik" (when an Eved Ivri he goes free). The message comes across loud and clear - there is no limit to giving; give and give again!
Counting the Omer
דברים טז, ט: שִׁבְעָה שָׁבֻעֹת תִּסְפָּר-לָךְ מֵהָחֵל חֶרְמֵשׁ בַּקָּמָה תָּחֵל לִסְפֹּר שִׁבְעָה שָׁבֻעוֹת:
Seven weeks you shall count ... from the time the scythe begins to cut the standing corn (ba'komoh) you shall count seven weeks" (16:9).
Baal HaTurim observes that the Torah refers to two countings, hinting at what Chazal have said 'It is a Mitzvah to count the days, and it is a Mitzvah to count the weeks'.
The Baal haTurim also points out that if we read the word "be'komoh" with a cholom instead of with a komatz, it will mean 'standing'. This hints at the Halachah to count the Omer standing.