Sacro Bosco di Bomarzo is a remarkable 16th century park in a wooded ravine below Villa Orsini. Giant mythical ogres, beasts and maidens were carved from bedrock and from enormous boulders. There is a massive figure of Orlando tearing the woodcutter limb from limb; a Mask of Madness, wild eyes staring and a mouth large enough to hold two adults; a gaping Mouth of Hell, a grotto cut into the rock furnished inside with a stone table. Here are some images, if you're interested:
http://gardentouring.fotopic.net/p51177417.html
http://gardentouring.fotopic.net/p51177405.html
http://gardentouring.fotopic.net/p51177421.html

If you have the time, and a car, it is interesting to visit Niki de Saint Phalle's Giardino dei Tarocchi along with Bomarzo. It's a few hours' drive to the west, near Garavicchio, in the southernmost corner of the Maremma. She's written that she was inspired by Gaudi's art in Barcelona's Parc Guell, but if you visit you will see it's also very much a contemporary re-working of 16th century Mannerist garden art, with echoes of Bomarzo.

She spent almost 20 years creating giant metaphorical structures covered in mosaics of glass and ceramics. You may be familiar with her work in other places, much of it created and sold to help fund Giardino dei Tarocchi, but to see a large collection such as this, her life's work, in light woodland of Holm oak, cork oak, olives and Mediterranean macchia, with constantly shifting patterns of light and shadow - it really is mystical, magical.
http://jmstudio.fotopic.net/p57611213.html
http://jmstudio.fotopic.net/p57611214.html
http://jmstudio.fotopic.net/p65835322.html

Both Bomarzo and the Tarot garden are surprisingly satisfying gardens to visit, given the initial impression you might have of a day out at Disneyworld with the children. In both, the placement of the structures has been thoughtfully worked out, creating surprise, discovery and delight - as all the best gardens strive to do.

By the way, I have visited all these gardens in both spring/early summer and fall and have to say each season brings special pleasures.

Yes, franco and thanks-Palazzo Doria Pamphilj if it has the hall that reminds one of Versaille.

This organization (for want of a better word; I think it's an "associazione culturale") offers occasional tours of the Palazzo Pamphilj (Embassy of Brazil). There was one on Tuesday, February 16.

For interiors in Rome, there's also the somewhat more accessible Palazzo Colonna, where the closing scene of "Roman Holiday" was set.

Sorry, Zerlina, I guess you forgot to post a link, or a name. Which associazione culturale?

So sorry. Here it is:
http://www.palazzidiroma.it/Le%20visite%20guidate.htm

What a great link, thank you! Even though the site seems ill-maintained (the "current" guided tours are for February and March 2010, and the "palazzo del mese" from March 2007!!), but well, this is an Italian website... and will certainly be useful for some rare opportunities to get into palazzi in the future. Thank you again!