It would be best, I reckon, to get your conference done first and travel after unless you have compelling reasons to do it the other way round. For accommodation around MiCo, try Best Western Hotel Astoria. For apartments, try Aparthotel Isola and Aparthotel Dei Mercanti.
Since you have about a week, I would suggest two countries and not three, which can be exhausting without doing much justice. Slovenia and Croatia would be good picks. Valid Schengen documents will serve all your travel. A possible itinerary is to take the train from Milan to Venice and change from there to a train or a bus to Ljubljana. In Slovenia, you must spend a night at Bled, where the lake and the castle should occupy you for a day, and perhaps you can stretch your tour to include Bohinj Valley as well. Also, take in the fascinating cave systems in southwestern Slovenia around the village of Štanjel, and then Lipica, where the gorgeous Lipizzaner horses have been bred for centuries.
For serviced apartments in Ljubljana, try The Lie Design Apartments and Apartments and Rooms Mešcanka. In Bled, check out Apartments Lana. There are more listings at bled.si/en. See slovenia.info for traveller information.
For Croatia, I suggest you spend your time on Zagreb, Split and then Dubrovnik from where you can fly home. Surface transport is easy in both countries and it’s a wonderful way to experience Europe: you can take a train from Ljubljana to Zagreb, a bus from Zagreb to Split (both journeys are accomplished under six hours each). To move from Split to Dubrovnik, I’d say take the ferry just for the variety of it, and it’ll get you there in five hours. For service apartments in Zagreb, try Apartments Lucky Place and Irundo Zagreb; for Split, see Apartments Varos and Villa Domina; and in Dubrovnik, Apartment Kesovija and Celenga Apartments.