Hi Juliette, Actually, even "I will decide for" is also a little "off". Please don't take offense.
Maybe: I will opt
As far as your query goes, Americans aren't huge tea drinkers, and most would not spike their tea, so that's a toughy. On that note, "spiked tea" might work, and would be understood, but may be too colloquial.
"lager with lemon soda" is not so good. That would not really be understood by most Americans.
"lager mixed with lemon-lime soda"
is maybe better.
Most Americans don't fully understand what a "lager" is, so you might opt for:
"beer mixed with lemon-lime soda"
If you can name the brand, to make it more readable: beer mixed with 7-up, beer mixed with Sprite.
"hard cider" is good, but I guess that was approved already.
This summer, I was drinking Summer Shandy
from Potosi Brewery in WI. "Shandy" is not unheard of in American English, but it's regional, and the Potosi Brewery is very crafty with their beer names. But as far as American beer culture goes, and Wisconsin is a good place for that, they are pretty out there:http://www.potosibrewery.com/
That's purely for your reading pleasure.
EDIT: I just noticed on their website that it was the "Steamboat Shandy", not "Summer Shandy". My bad.