First, go to the library & get a book or 2 on presenting your case in small claims court. I like Nolo Press books for clarity & practical detail. Then, visit the court & judge you'll be before & watch a few cases. See how much time the judge gives each side & what kinds of behavior tick off the judge.
In court, give the judge a road map, "Your honor, first I'm going to present my case for wrongful retention of the security deposit, then I'm going to present my case regarding the breach of contract when LL refused to remburse me for improvements he asked me to make."
Your stronger case is the wrongful withholding of the deposit, so lead with that. Present the lease & the law on deposits (actual copies of both with pertinant phrases highlighted), the initial condition of the unit, the 2 walk-throughs & the video/audio tape, and the 3 week letter & countersuit. The letter was insufficiently specific to satisfy IA Code 562A.12. Also, that LL sent it to your new address proves you gave him your new address. You should practice your presentation in front of friends to get used to the flow. Also, figure out if it works better to present the 3-week letter first & then point out every lie in it during the video presentation.
Then, after you've shown what a bald-faced liar LL is and prejudiced the judge against him, present your evidence of the improvement agreements. "Your honor, I'm now going to move on to the breach of contract issue." Any written evidence would be best to prove there was a contract -- receipts from Home Depot, letters you sent to LL explaining your repairs & asking if LL wanted you to deduct the total from your rent that month or would send you a check. (You then argue that LL's failure to deny the agreement at that time was an admission by silence, since LL would have written back a 'what have you been smoking?' letter if there was no agreement.) Where possible, point out that the improvements were not the sort of thing a college tenant would make on his own (too expensive, not portable), & where they can be seen in the video tape.
Next, call your witnesses. Try to present their testimony in an order that matches your case presentation. That will stimulate the judge's memory of your entire presentation. Have them practice their testimony & make note cards to keep them on track. Remember, they can testify ONLY to what they saw & heard and what LL said. They cannot testify to what you told them happened or what you said about something.
Stick to the facts directly relevant to your case. The judge will let you take more time as long as you're presenting facts that will help him/her decide the case. If you start talking about how nasty LL was to deal with the one time you were late with rent in January because your student loans were late, the judge will cut you off and your credibility will suffer.
Hopefully, by the time you say "That's all I have at this time, your honor. I ask the court to award me actual damages of $xxx for wrongful retention of my security deposit and to award the statutory maximum punitive damages of $200, and $xxx for the improvements LL agreed to pay for, plus costs," the judge will be so disgusted with LL s/he won't believe a word of LL's testimony.
