One day at La Paz

#1:
That might be a bit optimistic for one day. Can be done of course, but 3 hours in minivan each way plus whatever you have to add for transport within La Paz just to spend a couple of hours in Coroico sounds like a little too much of a hurry to me. While Coroico is definitely worth a visit, but I'd spend at least one night there.

OP:
There's loads to do inside La Paz itself. If you're interested in museums and culture, there's the Tihuanacu museum just off the Prado, or the Coca museum on calle Murillo, or several museums (on traditional dress, the War of the Pacific and several more) on colonial Calle Jaen. There's a huge market full of cholitas selling veggies, meat and fruit and lots of other stuff in the area between La Garita/El Cementerio and calles Max Paredes and Rodriguez, especially on Saturdays and Sundays.
There's the Valle de la Luna , a very small park with strange, moonlike landscapes - although in my opinion it's being ruined by houses being built all around it. Just grab any minibus or trufi going to the village of Mallasa, it's just before there.
Mallasa is nice enough place to spend a day too, it's a bit further down the mountain (2800 m.a.s.l. if I'm not mistaken) and therefore has a warmer climate than La Paz itself. There's a park and a zoo and loads of paceños (inhabitants of La Paz) go there on the weekend to have lunch.

And if you want to dance on musica nacional+ and get drunk on lukewarm beer with the locals there's places like +La Gota de Agua on calle Illampu. There's more 'internationally orientated' bars too, of course. Try Sopocachi or the Zona Sur for those.

I would do a day trip to Tiwanaku. We really enjoyed the tour. Fortunately we had an excellent guide.

The suggestion for Tiwanaku is a good one, if you like to explore archeological sites. You don't have time for Corioco in the las Yungas region. But a simpler suggestion would be just take one of the city tours on the double-decker buses, and get off the tour bus at the Valle de la Luna, south of the city, to explore the rock formations.

Go to San Pedro park at 10;00 and do the walking tour of the city with the two young women wearing red hats. Very interesting with lot of information you won't find in guide books.

Tiwanaku 'not really much to see' - have to disagree there - I thought it was absolutely fascinating - as was the journey across the altiplano. Most tours get you back to LP mid afternoon so still time for a museum (my recommendations would be the Coca Museum and/or the Museo de Metales Preciosos aka Gold Museum) as both stay open til 7pm on weekdays.
Still time fo a bar or two.

Definitely interested in Tiwanaku! And I think my hotel is in the south district, so i´ ll visit some cafes there! Thanx everyone.

I would do the death road... It's so wonderfull to start the adventure high and cold, maybe rain... And in a few hours... Who feel like a few minutes because of the speed, the adrenaline... As you GO down it's getting hotter and the fauna and flora change... It's really worth the money and the time... You don't need to be very sportif, I'm not and I did it!