I'd like to paint our cinder block wall but wanted to smooth the surface first. What are my options besides installing sheetrock on a wall?
To get a smooth surface you'll need to either use plasterboard (or sheetrock) or plaster (mud? in the US).
Both are DIY jobs, but you need a lot of practice to get a good finish with plaster.
You could use an epoxy coating (similar to what you use on a garage floor) - they are thick enough to give a smooth coat on cinder block. You would need to first morter/plaster the seams to get them smooth. There are also concrete compounds intended for use on vertical structures (see concretenetwork.com for examples) - that can give a very nice appearance.
Plaster or sheetrock would be quite a bit less expensive though.