I guess this must depend on the specific exibition but I visited the Design Museum on Saturday and really enjoyed it.
The Design of the Year competition (might not be what it is called....) was excellent, showcasing new and innovative design-led approaches to solving diverse problems. That you could buy a decent number of the showcase pieces in the shop downstairs helps further set the Design Museum apart as a place where you can look at the future and then enjoy your own personal taster.
Pick a nice day and treat yourself. If you are interested in design and are willing to engage with the exhibits you can easily kill a couple of hours here. And when you're done you can chill on the Southbank, wander down to Borough market or hop on a Clipper to Greenwich and get your picnic on.