There's the so-called 'Wonnemar', a waterpark that offers free admission on your birthday. The waterpark is located in Neu-Ulm, Bayern, Germany.
For sporting swimmers there are 4 lanes in the 25m lap pool. Also available are a cave, an outdoor pool with a lazy river, a neck shower, a slide area with slides for all ages, children's play castle, a children's toilet, changing table, a cascade whirlpool, a wave pool, a saltwater pool, a passable with tire slide, a turbo slide, a black hole slide, gastronomy, a sauna, free parking and a separate baby-pool.
And of course the entrance at your birthday is free: all ages on presentation of an official identification with photograph get free admission (day ticket WONNEMAR completely). Subsequent granting of this special day is not possible.