Short amount of time, I know, so I've tried to trim out the impossible (seeing anything in France but Paris, seeing any of southern Spain). This is my first attempt at an itinerary. I wouldn't want to plan things out so much, but I want to hit San Sebastian for Semana Grande, and I know we're traveling when it's very busy. As you can see, Spain is a major priority, and if it were up to me, I'd probably spend the whole time there, but my husband is interested in going a bit north... so there are some compromises here. :) We love just wandering city and town streets... we want to spend some of our time out of major cities.. and we would like to spend at least some of the time hiking, and a bit of time on the beach. Museums are slightly lower on our priorities.
August 9th Arrive in Madrid in the morning, go to Segovia or Avila (mainly to start somewhere low-key to get over jet-lag)
10th Travel to San Sebastian (Although I've been wondering about making this part of the trip longer and trying to go somewhere else in northern Spain.. I would REALLY like to go to Picos de Europa, but I don't know if it's too much)
11th San Sebastian (Probably a day for hiking)
12th San Sebastian (Semana Grande starts! I really want to be there for at least a day of it... I made a post about it, but no one responded.. anyone been to this festival?)
13th Travel to Paris
14th Paris
15th Paris
16th Paris
17th Belgium (Not sure where yet... want to see some World War history sights as well as try some Belgian beer and/or visit a brewery)
18th Belgium (Do we need another day here?)
19th Holland (We have relatives right across the Belgian border, looking forward to just spending them with them and having them show us around a bit)
20th Holland
21st Amsterdam
22nd Flight to Barcelona
23rd Barcelona
24th Barcelona
25th Madrid
26th Madrid
27th Fly home
Main questions: Out of the larger cities, are there particular ones that you would argue for a day or two more or less, and why? Should Belgium just be an overnight stop? Should we make San Sebastian a base for hiking and the festival, or would it be better go somewhere more dramatic for hiking (Picos de Europa or the Pyrenees or something) and only go to San Sebastian for beach time and the festival?

Well it is hard to know really what to say. Usually posters want to spend 18 days in Spain and we give the advice not to travel so much, and here you are going all the way to Irene’s home country The Netherlands.
So to be blunt you are trying to fit in too much. What are you going to see of anywhere with only a day in each location? You don’t seemed to have written down the days you are allowing for travelling. How are you travelling? To get over jet lag you will want to sleep, not visit tourist sites.
As for you specific question about SS and the Picos. Do you want to take your walking boots just for one day? They must weigh about 1.5kg per pair. That eats into your luggage allowance. Then you will need some sort of rucksack and weather gear, water bottle etc. all of this is fine if you are walking the whole vacation.
The other thing to consider is the driving distance.
Google Maps
Something like 357km. You just cannot do that and walk in the time period you are allotting.
Maybe a rethink on the whole schedule is on the table!

Thanks for the input on Picos... I thought my travel days were pretty clear in the itinerary (one flight, train the rest of the way). I don't know what else to clarify. Not to mention that the only locations with one day are Segovia and Amsterdam, so I don't know why you're saying "a day in each location." Hiking would be an overnight trip max... It's probably more realistic just to take a 1/2 day hike or two.
Honestly having taken road trips across the U.S. and Canada, 357km is nothing. I've traveled nearly 4000 kilometers by car in a week and had a great time. Sometimes I think that this board is programmed with a script that replies to all itinerary posts with "you're packing too much into too little time."

Distance is one thing, but also consider the effect of the landscape on the road. 357 in a flat landscape takes 4 hours, in the mountain this takes a whole day. Never been to the Picos, but knowing spains roads a bit, it will not be flat and easy driving.

For Belgium
Choose one of the culture cities ( Gent, Brugge,Antwerpen,Brussel ). A day is good for walking around and get a feel the town and its people.
For war heritage, the In Flanders Fields Museum in Ieper ( you know it as Ypres ) is a must-do.
In Flanders Fields Museum
and it is close to Brugge, so that might be a good combination

You really can't fit Picos in.
You could do a good day hike from BCN tho, taking the 7am train to Ribes de Freser and the mini-train up to Núria, then walk back down. Also it's easy to walk in the mountains as a day trip from Madrid, tho probably too hot in August (but same could be true for Picos).
Meerkat,
We are sorry you don’t like the advice we gave. We feel sorry that you felt it necessary to condemn the advice too. What is the point in posting questions here if you feel you have the answers?
If you arrive in BCN on the 22nd then leave on the 24th that is really only one day. The rest of the other days will be spent getting to and from the airport plus flying. There will be no time to spend, on those days, sight seeing. Like wise for several other destinations.
Hope you have an un-demanding vacation! We will not offer you any further advice.

Thanks progress, suiko, very helpful... I haven't looked into hikes near BCN yet, definitely a good idea. I hope others don't feel like they can't share advice.... Ribeira, I think we travel differently and that's fine. But I would still like to know from whoever if there are cities where I should add or subtract a day. Maybe that makes my request for advice more clear... eliminating the countries planned isn't an option on this trip (has to do with a compromise between my husband and myself... better than not taking a trip at all!).