Aside from being fun, I practice more often and with more attention on the tunes I enjoy.
Simple tune I see used a lot as learning tool in on-line tutorials, once you have it down (under the fingers) start ramping up the tempo.
X: 1
T: The Britches Full Of Stitches
R: polka
M: 2/4
L: 1/8
K: Dmaj
|:d>e fd|ed fd|d>e fd|edB2|d>e fd|ed fa|d>e dB|BA A2:|
|:~a2af|ed ef|~a2af|edb2|~a2af|ed ef|d>e dB|BA A2:|
Also here’s a great link to some played tunes for some ideas, take you pick they all sound good.
Do note though, The Britches full of stitches is not all that often played in D but it will do as a exercise. Once you’ve tried that, play the same tune in G and then in A. All on the same D whistle ofcourse.
Don’t ramp up the speed too much, it’s not a mad speed sort of tune.
Good exercise, playing in A is awkward after getting the flow going in D 
Then you’ve a golden opportunity to bring them new tunes? The first week you play it on your own and maybe someone will ask the name or where you got it. The next week you’ll play it on your own and someone might recognise it. The third week someone will record it when you play. The next week they might try and join in. Before you know it, it’s become ‘Giles jig’ and everyone in Ontario is playing it.