I did a 4-day stay at the Gleneagles resort in Scotland (actually we stayed nearby in a B&B and I drove over every day for my horse stuff) and it was great. I signed up for two sessions a day and we did everything from flatwork to XC schooling (a big, bold, black retired Intermediate horse who calmly took me over jumps I was scared to even look at--wheee!) to driving a four-in-hand. It was great--a really fun part of that trip and I enjoyed doing something different every day.
I've been to Fantasia Adventure Holidays in Spain and had a fantastic time... Riding on the beach and in the woods, the trails were amazing, the horses were great and safe and it was overall a fantastic trip. I would definitely recommend it!
If you ride in Spain you will find lovely horses, some fantastic countryside, good food and a culture that has 'horse' all the way through. Portugal, even better horses! The Lusitano in, in my opinion is one of the nicest to ride and the Portuguese are lovely horsemen. The summers can be very hot- so go for either mountains or on the coast. In France, there is such a variety of terrain and activities and the riding tends to be well organised as it is well supported by the French govt. Depending where you chose to ride, it can be cool maritime, high mountain, mediterrean and hot, so pick according to time of year. Usually, good food, reasonable to superb accommodation (staying in Chateau in the Loire was a great week). France is one of the leading nations for endurance riding and they have good specialist tack and a good attitude towards the horses. Personally, I prefer Spain to Italy but Italy is great for riding through countryside that is redolent with history. The local population, however, is strangely unaware of horses and riding on roads or through villages can have some interesting moments. Riding above the Arctic Circle in Finland was fun: you can drink from any flowing water it is so clean. I would avoid Greece: absolutely no horse culture and that concerns me in terms of horse welfare. Having said that, most riding companies there are run by foreigners. Not Europe, but close by, Morrocco has some amazing trips: choose the time of year.
One point, from conversations with fellow riders over a glass of wine in the evenings, it is apparent that American riders pay a LOT more for their riding holidays than do Europeans - for the same package. In this era of internet and credit cards, it is worth looking around.
Some friends booked a trip to Europe & when they returned they were angry as they thought they were overcharged. When they got to talking about what the Europeans paid, they didn't realize the difference in USD vs EURO and the exchange rate. Some booking agents tack on a booking fee or bump up the fee because of fluctuations in exchange rates - sometimes it can mean the Europeans pay more, sometimes less. Some trips offer choices of hotel or B&B. Many Europeans opt for B&B while Americans opt for hotel - B&B options are usually a lot less.