To summarize the weather effects:
- Sunny Day gives Fire-type moves more power and Solarbeam doesn't need to charge
- Rain Dance gives Water-type moves more power and makes Thunder and Hurricane 100% accurate.
- Hail gives Blizzard 100% accuracy.
If you use Defense Curl, immediately following it with Rollout or Ice Ball will double in power.
If Fusion Flare and Fusion Bolt are used in the same turn, whichever one goes last will double in power.
If you use Charge then an Electric attack, that attack will double in power.
The others you mentioned I wouldn't say give you more power or anything but they are good combos. Check the best move combos question for more.