Hey TWA, CLE isn't the greatest airport for variety but it does have consistency...
I would say if you've got flexibility in your schedule, try to visit on a Thursday night (7-9 PM) or Friday evening during rush hour (5-7 PM). On Thursday night, if it's clear, you can often see five or more planes lined up on the flight path.
If the weather is exceptionally good on a Thursday night, and they are using the 5s, you might even see VFR landings on Rwy 10. Last Thursday, for the first time in my experience, I saw VFR landings on 10 at night
. I didn't think that was possible.
I would say afternoons and evenings are the time to be at CLE, since that's when the 757s are in town.
Thanks for the plug, LoneStarMike!