Hi fellow travellers!
Next month I am starting a trip to Cambodia and Laos, 23 days on the ground.
It is starting with 4 days in Siem Reap, then flying to LP for 4 days. After that I have no set plans. So many places have been suggested to me. LP back to Cambodia is by bus. T
he places I KNOW I want to include are: Battambang, Kep or Kampot, Phnom Penh in Cambodia; Champasak and Vientiene in Laos. Please add or subtract form this list as you see fit. I want to be able to dally here and there. I want some countryside thrown in.
This is the first of several posts you'll be getting from me. Also posting in Laos room.
Many thanks!
Lane in WNC

OK, a 23 day itinerary starting with 4 days in Siem Reap - I'm on it! Give me a bit of time to put everything together and I'll get back to you with your full itinerary. Do you have a mailing address where I can also send supporting collateral such as photos and videos?
SR 4 days
LP 4 days
Battambang 4 days
Kampot 4 days
PP 3 days
Champasak 2 days
Vientiane 2 days
But really you don't need a rigid itinerary, just an idea of the places you'd like to see and maybe the order in which you'd like to see them. a starting point, a finishing point and perhaps arranged accommodation for the first night. Travel can be a much more pleasant experience when you just play it by ear.
Though, on reflection, I'd probably pre book my Siem Reap accommodation.

Note that the above list of places allows no time for travel. It can be slow at times, especially when you're crossing between countries. As an example alone, LP to Vientiane takes a good days travel and not one I'd do at night.

I am not sure about the visa situation.
But if you leave Cambodia and then return, have you checked on the type of visa you will require?
In any case, there was a time when it would be a brave American who addressed his or her comrades as "fellow travellers". I remember it well.

I don't know why you are going from Siem Reap to Luang Prabang. It would make more sense to stay in Cambodia for first part then fly to Laos for the second part.
The Champasak / Pakse area is in southern Laos. It is a long way from anywhere else on the region and given the amount of time you have, I would probably leave it out.
Maybe you could go Siem Reap - Battambang - Phnom Penh - Kampot - Phnom Penh
Then fly from Phnom to LP or Vientiane, and spend the rest of the time in northern Laos.
I too was confused and suggest building some logic into your proposed route. Do not worry about countryside there is plenty just chose what sort - jungle, hills, mountains, flatish with rivers / lakes and so on
Many recent post on kep vs Kampot
could suggest many more places for you to visit but very hard at this stage