How is the Geas spell intended to operate?
Reading it it appears to be a compulsion with side effects should the victim be hindered by something beyond their control.
How I would like it to play is that there is no compulsion, just the curse should they not follow through.
Of course I'm asking this because I want to use it against the players, but don't see it as an enjoyable quest hook one way and the other way is handwaving rules which I houseruled that there would be no houserules for this campaign.
Well, the compulsion could be subtle and the PCs could assume they are just trying to avoid the curse side of things. In fact, a well crafted Geas can appear to be railroading by the GM when in fact it is the compulsion driving the players to make certain choices (mental note - use this in a game).
How I would like it to play is that there is no compulsion, just the curse should they not follow through.
Major curse should be sufficient for that, if you're satisfied with the kinds of effects that spell can inflict. It won't stack up to -8 on all ability scores, but it does more than enough to get the point across. Heighten to taste.
Edit: Unfortunately, major curse doesn't have blanket immunity for lower caster levels, but you can layer different curses to make it harder to use remove curse--you have to succeed at all curse DCs or none of the curses get removed.