Most of the Conjure creatures spells it doesn't really matter what creature you get.   With the notable exception of Conjure Woodland Beings summoning Pixies.

For all other Conjure spells, I typically let the player pick the creature.   But for Woodland Beings, I state that all creatures must be the same type, they choose the Max CR, but I (DM) rolls:

CR 2: Roll 1d8

1-3: Sea Hag

4-5: Nereid

6: Meenlock

7: Darkling Elder

8: Roll on CR1: table (note it says CR 2 or lower....)

CR 1:  Roll 1d6

1-3: 2 Dryads

4: 2 Quicklings

5:  2 Screaming Devilkin

6: Roll on CR 1/2 table unless you started with CR 2, in which case, roll again on CR 2.

CR 1/2:  roll 1d6

1-2: 4 Satyrs

3: 4 Darkling

4: 4 Valenar Steed (and I house rule that summoned creature is bonded to you)

5: 4 Valenar Hound (Summoned = bonded)

6: Roll on CR 1/4 unless you started with CR 1, in which case, roll again on CR 1.

CR 1/4:  Roll 1d12

1-3: 8 Sprites

4-6: 8 Pixies

7-9: 8 Blink Dogs

10-11:  8 Mites

12: roll on the CR 1/8 or 0 table: 

CR 1/8 or 0:   Roll 1d6:

1-2: 8 Valenar hawks (again, summoned = bonded)

3-4: 8 Boggles

5:  8 Killmoulis

6:  Go back to to the CR 1/4 table and re-roll 12 if you get them again.