Observe the galactic 21cm hydrogen line yourself! Its easier than it ever was! You need a horn, some filters, an amplifier, and a software defined radio (SDR).
The CHART* project puts students in the drivers seat to design and build. They have a quite nice design for a cardboard horn that does not require a can for the waveguide. Their web site has many video tutorials about horn construction, using gnuradio to make sdr programs, solving linux problems and so on. The CHART designs and website was developed in the LoCo group by Adam Beardsley (now at Winona State) and several students. *Completely Hackable Amateur Radio Telescope