On clothes, sure you have it all in hand so I guess you're asking about activities - if not, ignore what follows!
There are plenty of museums and galleries for a rainy day if you're into that sort of thing (I am). And there are fantastic paintings, mosaics, etc in churches too of course. Some - eg S. Clemente - are built on top of even older churches and temples going back to Roman days. The fantastic Roman open-air sites really need dry weather. As do such things as the open-air food markets.
One of my favourite galleries is the Villa Borghese - but it's really better to go there on a nice day because it's in a lovely park. I suppose the Vatican Galleries could also keep anyone occupied for a day though I'm not a fan myself - too many queues and crowds. Prefer churches and more quirky, smaller galleries, such as the Borghese, or the Doria Pamphilii.
My other favourite thing for a wet day - apart from shopping of course (Via Condotti area, all the usual suspects, very compact. Or try Via del Governo Vecchio for young designers) - is people-watching in bars and cafes. My best tip really, if not very original, is to get the latest Time Out guide, which has good listings for restaurants, bars and cafes as well as galleries. I've found I can easily construct a day in any weather going from one cafe to another, with shopping and culture included as desired!
PS If you speak French and Spanish you won't find the language too hard.