I'm heading to Spain this summer for 5 whole days in Madrid and Barcelona. I need some ideas on the must see sites, restaurants and hidden gems of the city... any suggustions?

For Madrid I would suggest that you should not miss the Palacio Real or Royal Palace, small by European standards, it is none the less a little gem. Also the Plaza Mayor is worth a visit as are some of the restaurants there. We found the Lazaro Galdiano Museum well worth a morning.

For Barcelona, a much smaller city, we enjoyed the Las Ramblas Cathedral and took the funicular or cable railway up to the Castell de Montjuric, which is interesting and gives excellent views. The Cathedral of Sagrada Familia may not be to everybody?s? taste, it is certainly not to mine, but it is interesting.

Go to the Restaurant Pitarra in Barcelona (near the Gothic quarter). Authentic Catalonian cuisine, complimentary apple schnaps with your coffee after dinner (I guess, it wasn't on our bill

There's also a great tapas place here and I don't remember the name but it is on Las Ramblas.

I second the Madrid Royal Palace (and the gelato sold nearby!!!) and the restaurants around the Plaza Mayor; Las Ramblas in Barcelona (just don't play the shell games) and the gothic cathedral...Sagrada Familia if you're into Gaudi and Parc Guell is you're REALLY into Gaudi! Tapas places all over Barcelona...I thought I'd hate them but ended up loving them.
If you haven't been before remember they eat LATE 9Pm onward. Hot and dry in Madrid; hot and humid in Barcelona..enjoy!

Do a day trip from Madrid to Toledo, an amazingly intact medieval town peppered with old churches. Nicer than anything in Madrid, a smoggy busy modern city overrun with muggers and bag snatchers. Be careful even in main tourist venues, so many folks i know have been mugged or pickpocketed in Madrid. One doctor said at this convention nearly everyone had been pickpocketed. why this may have been byperbole, Madrid, along with Barcelona are the two worst cities in Europe it seems for crimes against tourists. my friend was mugged at 7am on a Sunday morning going to Atocha train station, thief tried to get her day pack off and she ended up in gutter trying to get the bag off too because it had nothing of value in it. but neither mugger or my friend could get in off for a minute and the struggle continued. meanwhile shopkeepers opening shops a few feet away did nothing. Be on guard in Madrid and Barcelona. Besides Toledo, 45 minutes by train from Madrid, Avila and Segovia are also great old-world cities reeking in old-world looks that is nearly totally lacking in Madrid.

I just got back from an Amsterdam-Paris-Madrid-Seville trip and here are my suggestions for Madrid:
First off, the crime/pickpocketing thing is way overstated. Just exercise the common sense you would in any big, crowded city and you'll return with your wallet and passport intact. People who tend to be intimidated by big cities always whine incessantly about such things.
Make sure you visit as many grand plazas as possible in Madrid - all of them are beautiful with majestic fountains. (Plaza Colon, Plaza Arguelles, Plaza Carlos V etc etc)
The Plaza Mayor is a natural, as is the Paseo del Prado (the street on which the Prado is located). The Retiro park is stunning, but ignore the gypsies trying to read your palms for some euros.
Just walk up and down the bustling, teeming streets like Gran Via. Check out the pedestrian area between Puerta Del Sol and Gran Via where the El Corte Ingles Dept. store is located - it's teeming with people day and night.
Also, check out modern Madrid architecture, such as in the Plaza Europa area. For a great and cheap (1 euro) view of the city, go up in the Faro de Madrid, near the university.
You'll have a great time, but be prepared for the heat. I was there from June 25-29 and the coolest high temp. was 96 fahrenheit.