I'm assuming you don't plan to rent a car.
How practical it will be to stay in other towns in those regions (as opposed to less-touristy neighborhoods in those towns/cities) will sometimes depend on your sightseeing plans. For week-long stays with multiple day trips, it can get annoying to have to start out every day with a train or bus trip to the transportation hub and then often have to immediately transfer to another train or bus to get where you really want to go.
I favor long stays myself, and I always choose the big city as my base, but I like cities, and I spend a lot more time in museums than the average visitor. Plenty of folks much prefer the more intimate environment of a smaller place.
Barcelona has a lot of sightseeing in the city itself, but there are lots of good side trips available. How do you think you might spend your time in that area? I sometimes recommend that folks spend a few nights in Girona, north of Barcelona. It does get a lot of tourists itself, but many are just daytrippers. Girona has a feel that's very different from Barcelona, and it's very well-positioned for side trips to the Dali Museum in Figueres, the attractive former fishing village of Cadaques and medieval Besalu.
In Barcelona you might like to look at the neighborhoods of Eixample and Gracia. The local subway system is very useful, and of course there are also buses.
Nice is the transportation hub for the Riviera. There's a very handy regional train line running along the coast, so you can visit the coastal towns quite easily from a base other than Nice, as long as you're staying near a train station. However, the towns up in the hills usually require a bus trip, and the buses tend to fan out from Nice. Seven days is long enough that you'll probably be taking a bunch of side trips, so staying in Nice itself would be more efficient. It's not essential, though, if you want yo be in a smaller town. Two options others have mentioned are Villefranche-sur-Mer and Antibes. Do look at a map, though, to be sure your planned side trips won't be excessively long from the base you're considering. The coastal trains are not fast. I wouldn't choose to,stay somewhere east of Nice if most of my sightseeing was going tombe west of Nice, or vice versa.
Seven nights would be a very long time in the Cinque Terre. Are you hard-core hikers? The towns are totally swamped during day tripping hours, and there's not much sightseeing that doesn't involve hiking. I think a week would feel like a long time in any of them if you weren't heading uphill to hike just about every day. What are your plans in that area? We need that information to suggest alternatives.
A week in the Lauterbrunnen Valley suggests that you are hikers. I think folks will be in favor of your staying in Lauterbrunnen or higher up if your main goal there is to hike. But that's a very popular area, and it may be getting late to find lodging there for the peak summer period. Others will be able to advise you better.
I don't know Bavaria well at all. Other posters should be able to make suggestions if they have an idea of how you plan to use your time there. Munich is not a city where I'd expect to find a week's worth of sightseeing, but I could be mistaken about that.
