You can do one "signature" cocktail -- which is great for beachy-themes anyway. Pick your favorite (or make up a new one).
You could also combine two themes--Canada & Mexico into something like "Snow on the Beach" (Baileys in hot chocolate or coffee with whipped cream would work for that or even a "dirty mother" or white russian.) or "Winter Heat" with something like a cinnamon schnapps drink. Or a Frosty Bikini (anything frozen or on the rocks--something like a cosmopolitan would work for that).
Just think of beachy names & put them to work in your food & drinks--Lifeguard, waves, surf, seashells, starfish, boogie board, flippers, fish, coral, sand, sun, and so on.
You shouldn't have to spend a lot on decor, either--borrow beach towels--chloe's right they are great decor!, pick up some things at a dollar store. Plastic glasses you can write guests names on & your birthday or the party date make good party favors. If you can draw, add some seashells or something. You could do the same kind of thing with mugs if you choose a hot drink. (And, for a stirrer, you can stick in a cinnamon stick or make plastic spoons dipped in chocolate which are really cute, and inexpensive.) Flip flops are another great favor--you can maybe get them at the $1 stores.
Chloe's idea of bringing in lawn furniture is great, too -- instant decor!
And, since you like the idea of being outside, if you can get everyone to wear swimsuits (maybe under their clothes?)you can do tons of fun games--even a snowman building contest--jazzed up by the fact that everyone is in swimsuits! It would make for great pictures, too!
Have fun!
