N4
たらどう
Why don't you...?, What if you did...?
公園に遊びに行ったらどうですか。
Why don't you go to play in the park?
Explanation
「〜たらどう」 means 'how about …?' or 'why don't you …?'. You suggest that the other person do something to solve the problem at hand. The tone varies:
1A friendly suggestion between friends: 「少し休んだらどうですか」 (Why don't you rest a bit?).
2Urging, or mild reproach: when the other person keeps hesitating or complaining it can sound sharp: 「文句があるなら、自分で言ったらどう？」 (If you've got a complaint, why don't you say it yourself?).
3In speech it shortens to 「〜たら？」: 「病院に行ったら？」 (Why not see a doctor?).
Mixed up with
Do not use it with a boss or elder. Even with 「〜たらどうですか」 it sounds like telling them what to do from above.
To suggest something to an elder you can say 「〜てはいかがでしょうか」, 「〜なさったらどうでしょうか」 or 「〜てみるのはいかがでしょうか」.
To suggest not doing something, use the ない form: 「行きたくないなら行かなかったらどう？」 (If you don't want to go, why not just skip it?).