The Ritz Carlton in Berlin is very nice and good value. The Grand Hyatt round the corner is also very plush - you may want to try a night in each. (In fact, if Hyatt's Faster Free Nights comes back, do 1 night in the GH, 1 in the FS and 1 back in the GH!!)

Lisbon is cheap but, apart from the Orient Express property out of town, there is only the FS in the centre, I think. This is a horrendous 1970's building but the hotel itself is well loved.

The FS in Istanbul was cheap earlier in the year - you'd need to check the Summer rates. We were going to go for Easter but switched to Nice in the end.

Bilbao - the Domine is exceptionally good in terms of its design, but falls short of the full luxury experience in some areas. It is certainly well worth a visit, however.

The FS in Prague and Budapest are both meant to be fantastic, and if you've never been to these cities are certainly worth considering.

Slightly off the wall, but look at the Hotel Kamp in Helsinki (Starwood Luxury Collection). They have some good Summer rates and Helsinki is lovely when the sun shines. You can also get the ferry across to Tallinn for the day, tho' its better to stay one night for the nightlife. The Kamp is easily the best hotel in Scandinavia and a real credit to Starwood.