Well, let’s see. I can identify:

(1) faulty parallelism;
(2) the semicolon that appears to make the structure less smooth.

As you are trying to contrast two things in this sentence, you are using quite an abrupt structure. I think you can improve the sentence if you use the same form in the second part, the gerund, and I would recommend replacing the semicolon with ‘but’ because in this way your sentence sounds better. Also, remember they do not normally use any punctuation marks with ‘but’ because it is natural for people to stop when they pronounce it. So your sentence will need to be as follows:

Doing the lab tests was fun but writing the report was complicated.

It seems you have not heard about the rule of parallelism yet, this rule requires that you use the same grammar forms in parallel constructions, so you will need to restate the sentence so that there is the same verb form in every part and you need to know that gerund is more natural for this type of sentence.

The main problem here is the faulty parallelism. Thus, you can have two choices here:

1. Doing the lab tests was fun; writing the report was complicated.
2. To do the lab tests was fun; to write the report was complicated.

Each sentence will work grammatically, but the first one with the gerund is more commonly used. You need to understand that parallelism in similar structures requires the use of the same verb forms.