Mana Leak

Counter target spell unless its controller pays {3}.

Consider

Surveil 1. (Look at the top card of your library. You may put it into your graveyard.)
Draw a card.

Sleight of Hand

Look at the top two cards of your library. Put one of them into your hand and the other on the bottom of your library.

Blow Off Steam

Blow Off Steam deals 1 damage to any target. (Then exile this card. You may cast the creature later from exile.)

Frolicking Familiar

Flying
Whenever you cast an instant or sorcery spell, Frolicking Familiar gets +1/+1 until end of turn.

Frolicking Familiar // Blow Off Steam

Lightning Strike

Lightning Strike deals 3 damage to any target.

Scry 1. (Look at the top card of your library. You may put that card on the bottom.)
Draw a card.

Thundertrap Trainer

Offspring {4} (You may pay an additional {4} as you cast this spell. If you do, when this creature enters, create a 1/1 token copy of it.)
When this creature enters, look at the top four cards of your library. You may reveal a noncreature, nonland card from among them and put it into your hand. Put the rest on the bottom of your library in a random order.

Grove's Bounty

Distribute X +1/+1 counters among any number of target creatures you control. (Then exile this card. You may cast the creature later from exile.)

Elusive Otter

Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)
Creatures with power less than Elusive Otter's power can't block it.

Elusive Otter // Grove's Bounty

Crash Through

Creatures you control gain trample until end of turn. (Each of those creature can deal excess combat damage to the player or planeswalker it's attacking.)
Draw a card.

Stormcatch Mentor

Haste
Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)
Instant and sorcery spells you cast cost {1} less to cast.