What type of garage heater should I connect to my 30A circuit?
Common electric convection baseboard heaters are ridiculously cheap, in the neighborhood of $50 for a 2000W unit and $15 for a 30A thermostat.   

Select your heaters so they total up to less than 80% of 7200W.   

That plus some EMT conduit, a few junction boxes and some red 10AWG THWN wire to hook it all up, done.