Some of the good oneday trips from bangalore are
Mekedhatu - 100+ kms (kanakapura road) - Scenic spot/waterfalls
Savanadurga - 60+ kms (Magadi road/30+ kms from banyan tree) - Hill Fort
Shivaganga - 80+ kms (Tumkur Road) - Hill Fort
Devarayanadurga - 90+ (Tumkur Road) - Hill Fort
Madhugiri - 100+ kms (Tumkur Road) - Hill Fort
Ranghanathithu - 70+ kms (Mysore Road) - Bird Sanchuary
Shivanasamudra - 70+ kms (Mysore Road) - Waterfalls
Muthathi - 80+ kms - Scenic spot on river kaveri
Hoganekal - 150+ kms (Tamilnadu) - Waterfalls
Talakadhu - Scenic spot on river kaveri and a virtual desert
there are 7 shiva temples which used to get covered by sand every year
that is when the kaveri was in full flow.
There are eight durga's or hill forts surrounding bangalore. Hope to cover them all in the near future.
And guys dont go for swim in Manchinabele dam, many people have drowned there.
The best places to cool off will be Mekedhatu/Talakadhu/Hoganekal/Muthathi
all on the kaveri river.