BombSpawner.cs

using UnityEngine;

public class BombSpawner : MonoBehaviour
{
    // a public object array for which objects to spawn
    public GameObject[] obj;
    //min and max times for another spawn
    public float spawnMin = 3f;
    public float spawnMax = 2f;

    void Start()
    {
        //start spawn 
        Spawn();
    }

    void Spawn()
    {
        //get random number
        float rand = Random.Range(0, 1000);
        //if random number is greater than 700 make a bomb
        if (rand > 700)
        {
            Instantiate(obj[Random.Range(0, obj.GetLength(0))], transform.position, Quaternion.identity);
        }
        //invoke spawn at random time interval between min and max
        Invoke("Spawn", Random.Range(spawnMin, spawnMax));
    }

}

This is the main script that I used for spawning my in-game objects, such as bonus points and bombs, among others. Using Random.Range, I made the object with this script attached to it spawn at random intervals.