The Planetarium is Adler Planetarium, I haven't been there in ages, but it is cool.
Museum of Science and Industry is pretty far south, but you can take public transportation to get there .. and it is easy to spend a whole day there, so be careful!
Hotels might be cheaper in Rosemont or near O'hare airport, generally, both are pretty safe/secure areas... so you may want to keep that as a plan b on the back burner...
I've never stayed at a hotel in the city since I've always lived in the suburbs just an hour or so away...