My father was a wandering Aramean (26:5)
These are the opening words of mikra bikurim, describing how God took out us out of Egypt and gave us the land which produced these bikurim. But what is the point of mentioning that our ancestor was once a wanderer?
I think it's to create a contrast between our initial state (when we were wanderers and did not have a land) to our current state (where God has given us a land).
Vidui Maasrot (26:12-15)
Chazal refer to the second mitzvah of the parsha as "vidui maaser" (nowaday commonly known as "vidui maasrot"), though the Torah does not give it a name. It only says "you shall say" ("veamarta"), followed by your declaration that you have given maaser ani and thus have kept the mitzvah (and not done one of several possible forbidden actions), followed by your request that God bless you.
The word "vidui" in our minds suggests a confession of sin. But in this case you are "confessing" only that you have done good things, not bad things. So why do Chazal call it "vidui"?
One possibility is that "vidui" in Chazal's language can refer to any declaration, not only to an admission of sin. I found one source which fits this: Tosefta Bikurim 1:7 says that both bikurim and maaser ani require "vidui". Since neither of the two declarations seems to obviously allude to sin, one may presume that "vidui" in this source simply means "declaration". That said, this source seems to be the exception. It appears that in the vast majority of cases where Chazal (or the Torah) mention "vidui", they are referring to confession of sin.
If the word does imply confession of sin, I want to suggest what that sin might be. Elsewhere (Shemot 23:15) the Torah requires that when we visit God we must "not appear before [Him] empty-handed". Technically this mitzvah only applies to the pilgrimage holidays. But like many mitzvot, it is easy to identify a sensible broader idea which lies behind the specified application. In this case, the idea is that one visiting God should bring a gift with them, representing their love of God and submission to God.
The mitzvah of maaser ani follows an unusual schedule, dictated by the shemitah cycle. In years 1, 2, 4, and 5 of the cycle, one brings maaser sheni - holy food - to eat in Jerusalem. In years 3 and 6, maaser ani takes the place of maaser sheni.
Vidui maasrot is recited on an occasion when, despite the absence of maaser sheni that year, you are still visiting the Temple. As mentioned above, I think there there is an expectation to bring some kind of gift when visiting the Temple. But in this case, the gift you would normally be bringing (in 4 out of the 6 non-shemitah years) is conspicuously absent.
Therefore, I think vidui maasrot is an acknowledgment of this lack of gift. You "apologize" for your lack of gift, and then explain the good excuse you have for not bringing it - that God told you to do something else with the food. This acknowledgment is a "confession", not of having broken a commandment, but of failing to meet the expectation that normally goes along with such visits to God.
When you cross the Jordan, you shall erect these stones, which I command you today, at Mount Eval... You shall slaughter shelamim-offerings and eat there, and rejoice before Hashem your God. (27:4-7)
The mitzvah of "simcha" is a part of the three pilgrimage holidays, and also of other gatherings at the Mishkan/Temple mentioned in the Torah, such as this one. A common question is, how can you command someone to be happy?
One answer is textual: you are not commanding them to be happy, but rather to hold a celebration.
Another answer is that the activities involved are ones that naturally promote happiness. This is true on a number of levels. The meal is a source of physical enjoyment. The vacation from work is inherently pleasant. The group and family bonding of the event are good for mental health. The affirmation of common values gives people a sense of purpose. And finally, occasions of "simcha" are generally centered around thanking God for one's well-being, and psychologists tell us that feeling gratitude is one of the most effective ways of becoming happier. All these factors combine, not to guarantee that a person celebrating will be happy, but to make it much more likely.