行ったら楽しいって英語でなんて言うの？

"行ったら楽しい"
- It's fun if you go.
- When I go it's fun.

「旅行はあまり行かないけど行ったら楽しいです」
- I don't go on many trips but when I do they're a lot of fun.
- I don't go on a lot of trips but they are fun when I do go.
- I don't go on many trips but the ones I do go on are fun.

「行ったら楽しい」は
ーWhen I go, I have a good time.
のように言えます。

例：
I don't really go travelling very often, but I have a good time when I do go.
「あまり旅行に行かないが、行ったら楽しい」

I don't travel much, but I really enjoy myself when I can go.
「あまり旅行に行かないが、行ける時はとても楽しむ」
I really enjoy myself when I can go. という言い方もできます。

ご参考まで！