Some years ago I bought a pair of Nikon Travelite 8x25 binos for the bag and they have been excellent-sturdy,very,very light and nice to use.(I think they,or similar are still made).Basically for general use but good for seeing fish move(as long as the distance is reasonable)and bird-watching etc.
Also bought a pair of 10X25 for trying to spot fish moving at greater distance but the x10 magnification was hopeless with such a small lens size of 25.Obviously if you increase this to(say)35 then the price usually goes through the roof and up goes the weight so it does depend to some degree on what you have to spend.
So I would say no bigger than x8 mag.very light,waterproof if possible but this pushes the price up!
I am certain many makes would fit the bill but I chose Nikon because weight was the main issue and they were the lightest by far anda professional told me Nikon lenses are second to none.
Not too many specialised shops about nowadays so I went to the RSPB Centre at Sandy,Bedfordshire who have a good selection and who let you walk around the Reserve for a while to try them out.However you may buy them cheaper online.
One thing for sure-I wouldn't be without mine!(I'm also told you can see what bait people are using on the opposite bank....)