I agree TellyN, we find there's plenty for us to do in Tekapo for a weekend, so a day and night stop would be easily filled. We were there for the weekend a few weeks ago, sad to see the Astro cafe on top of Mt John was closed, hopefully it will reopen again before too long (if it hasn't already). It's a favourite stop of ours when we're staying in Tekapo. Admittedly if the weather is bad the options for things to do in Tekapo can become more limited, but there's always the hot pools. But otherwise in winter you've got skiing, ice skating, tubing, walking (some nice trails by the lake or up to the top of Mt John), biking (some nice trails in the area, including flat family suitable ones), cafe's, star tours, hot pools, in summer you've got all that and boating, minus the skiing/tubing/skating. Plus the touristy thing of visiting the church but that doesn't fill much time.
I sound like a tourist advert, but I do find it slightly bewildering when i read advice on this forum that people shouldn't bother with Tekapo.