Alder, cherry, peach are nice mild sweet smoke flavors that - for fish, or pork can be really nice.
If you have some 100% flavor wood pellets you can put them in an Amazen smoker ( don't think I would use them as fuel for the grill...without blending them with oak for burn consistency) they can be really good to experiment with.
Now - we have a new sponsor here on LTBBQ- Milt's Original Gourmet BBQ Pellets - and MIlt makes some amazing sounding flavors - and they are 100% flavor wood. Makes a difference. Check out his site!