Main goal was to see wildlife, especially animals like elephants, giraffes, rhinos, lions, and zebras, and nature. October in Namibia is the end of the dry season, so it's a good time to see land mammels near the water holes.

Etosha - This national park is full of absolutely amazing wildlife. Each camping area has it's own waterhole which you can sit and watch the animals that come to drink water. We spent 4 days here, and loved every one.
Namibia Craft Centre in Windhoek - has both traditional and modern crafts - many crafts-person owned shops under one roof - set prices w/ no haggaling, so you don't have to stress over negotiation. Also has a nice coffeeshop in case any of your group doesn't enjoy shopping :)

Dune 45 - beautiful dune to walk up the ridge, or playfully jump down the side - it can be quite popular, especially at sunrise. You can drive right up to the edge, so it's quite accessible.
You can't drive to the other dunes, but we think you might be able to walk to them. So, I'd recommend looking into that if you want to avoid the crowd.

Deadvlei - beautiful place for photographers! Striking colors. Fascinating place.
Waterberg Plateau - Enjoyable good stop on the way between Windhoek and Etosha, but not a "must do". Enjoyed hiking & scrambling up to the edge of the plateau to get a good view, especially since you otherwise spend so much time in the car on safari. Also enjoyed the guided game drive on top of plateau to see animals including viewing from inside a hide. Nice swimming pool to cool off and nice restaurant in historical fort.

hot air ballooning near the dunes - We had expected that we would go over the dunes, but due to low wind we mostly went up & came back down. Obviously they can't control the wind, but it sounds like this was actually a pretty typical experience and it would have been nice to have the right expectations going into the experience. On the plus side, we saw a stunning sunrise and enjoyed the skill of our balloonist who landed us on the back of the truckbed. The desert breakfast was quite fancy with table clothes, champagne, and a tasty array of food. In short, this was a very enjoyable experience, but if you are watching your budget or are only going to do a hot air ballooon once in your life I wouldn't splurge for it here.
Erindi - We enjoyed our first two days in this game reserve, it was fun to do our own game drive in our own vehicle, and Elephant camp had more amenities than some of the later camps. That said, we would have been disappointed if we'd seen this after Etosha and it felt a little strange that they were feeding the wildlife (although it was a severe drought so food was scarce for the grazing animals). Waterberg plateau is also on the way to Etosha from Windhoek, and I'd recommend that stop instead.
Okahandja craft markets - If you love the craft market experience and don't mind a good bit of hassling, this might be for you. Local boys "help you park" in the only parking lot and then persistantly ask for money when you leave. Almost every shop-keeper will be insisting that you to come into their store, staying with you as you look from thing to thing, and fairly aggressively pressuring you to buy. And of course, you have no idea what the price is for anything. At the same time, they do have many beautiful crafts and seemingly reasonably local craftspeople. We finally found one shop where we could browse in peace, and ended up buying a few things there. Note - we went to the northern market. There is also one on the south end of the town.
If you prefer set prices and less hassle, check out the Namibia Craft Centre in Windhoek.
Wild Dog Safaris - we used this travel agency to rent a truck and to make many of our camping reservations. I would NOT recommend them, unless it's the only way you can get camping reservations in Etosha. Etosha is a national park, but often you can't buy camping or hotel reserations directly from the park because agencies like Wild Dog have already purchased them to resell to customers later. Wild Dog was extremely difficult to communicate with, expensive, and a month after our trip we got a bill for the rental car which we'd already paid for. Try Cardboard Box Travel instead.
Backpacking on Waterberg Plateau - Due to lions and other predators, you spend a lot of time driving rather than hiking while on safari. One exception is that you can do a 3-day backpacking trip on the Waterberg Plateau, where you can see giraffes, rhinos, zebras, and many other animals. Yes, there are leopards, but they hunt at night when you are staying in a (basic) shelter.
Khaudum - This game reserve tucked in the north-eastern corner of the country is for those who want more of a classic Africa safari experience: no fences and true 4wd sandy "roads". Heard that it's an amazing place to see elephants. We wanted to go once we heard about it, but couldn't quite make the timing work.

Oct 11: Windhoek, Tamboti Guest House - pick up camper truck at airport from Hertz
Oct 12: Shopping & Drive to Camp Elephant at Erindi
Oct 13: game drives at Erindi
Oct 14: drive to Halali camp in Etosha National Park
Oct 15: game drives in Etosha, stay at Halali
Oct 16: game drives in Etosha, stay at Okaukeijo camp
Oct 17: game drives in Etosha, stay at Namatoni camp
Oct 18: drive to Waterberg Plateau
Oct 19: hiking at Waterberg Plateau
Oct 20: morning game drive at Waterberg, drive to near Windhoek on route to dunes, shop at Okahandja craft markets, stay at Monteiro Camping
Oct 21: drive to Sesriem camp near Sossusvlei
Oct 22: hot air ballooning in the morning, stay at Sesriem camp, visit Deadvlei and Dune 45
Oct 23: sunrise at Dune 45, drive back to Windhoek, stay at Monteiro Camping
Oct 24: shopping & coffee at Namibia Craft Centre in Windhoek, catch flight at airport

4 people traveling together: 2 in their late 60's/early 70's and 2 in their 30's Rented a 4wd truck with 2 camping tents on top, mostly cooked own meals. More images of this type of truck w/ camper here.
Not necessary, but would make things less stressful and would give more options for where/when you stay in Etosha.
We planned our trip just 2 weeks before we started. But, would definitely recommend planning earlier than that in order to have a less stressful planning experience. Things that require reservations and are hard to get at the last minute included: 1. renting a camping truck (many places were sold out), 2. getting reservations at campsites in Etosha, and 3. backpacking at Waterberg Plateau
Definitely try to stay in Etosha itself, especially at Okaukeijo camp or Halali camp. If Etosha says there is no more camping or hotel reservations available, try going through a travel agency. Often there are campsites that are available, but they've been pre-reserved by the travel agencies.