You can cast any spell that does not require you to see and that has a valid target
But before we get into that, a special service announcement:
You cannot cast spells in wild shape
See page 67, PHB under the rules that apply to wild shape
You can’t cast spells, and your ability to speak or take any action that requires hands is limited to the capabilities of your beast form.
You are level 6, so this applies to you. It won't change until you reach level 18 and gain the "Beast Shape" feature that says
Beginning at 18th level, you can cast many of your druid spells in any shape you assume using Wild Shape.
What spells can you cast once you are in normal shape?
If you give up your wild shape to cast, as per this Q&A, you are most likely blinded and restrained - it depends on what the swallow ability of the monster says, but most have this text:
While swallowed, the target is blinded and restrained, it has total cover against attacks and other effects outside the
So you cannot cast spells that require you to see the target, but apart from that the condition imposes no limitations on spellcasting itself.
You need to check each spell for what the requirements for sight and target are.
Flaming sphere would not work, as it says:
A 5-foot-diameter sphere of fire appears in an unoccupied space of your choice within range
And inside the creature there is no unoccupied space, and you do not have a clear path of effect to any space outside the creature, due to total cover. So while you technically can cast the spell, there is no valid target. (p. 201, PHB):
To target something, you must have a clear path to it, so it can’t be behind total cover.
Wall of Water will also not work as it says
You conjure up a wall of water on the ground at a point you can see within range.
And again, there is no point on the ground that you can see (and you cannot see, to begin with).
How will the spell work once cast?
Tidal Wave can be cast as it has no such limitations, but it is more difficult to determine what then happens. Its area of effect is
You conjure up a wave of water that crashes down on an area within range. The area can be up to 30 feet long, up to 10 feet wide, and up to 10 feet tall.
While this is not one of the standard areas of effect (cone, sphere, line, cube or cylinder), it still is an are of effect. The targeting rules say (p. 204, PHB)
If you place an area of effect at a point that you can’t see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.
You have total cover inside the creature so it provides an obstruction to any point outside creature, and any point outside likewise has total cover. That means if you pick a point of origin on the outside, it instead will end up on the inside, and the area would expand from the inside. Area effects however cannot expand either, if there is no clear path:
A spell’s effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn’t included in the spell’s area.
Since the obstacle here is also a creature that can potentially be affected by the area spell I’d check with my DM as to how they want to handle this unusual situation, and if they want to apply the effects to the creature too, or only the inside of the creature.
