Santa Monica Pier is always a good date idea anytime.
The Heal the Bay Aquarium is open daily except Mondays and some holidays: 2-5 PM on Tues-Fri and 12:30 - 5 PM on weekends during winter (Oct-Mar). The admission fee is $2-5 donation. There are many options for parking as low as free for first 3 hours (check website for map).
For Valentine's weekend, the aquarium is sponsoring "Whale of a Weekend" with family-friendly activities all day. Be aware that it's a kid-friendly place all of the time, so this is something fun to do, but not necessarily the most romantic. If you're lucky to have kids, admission for each child is free with a paying adult during this special event.
I suggest going on Sundays, so you can also grab a coffee and bite to eat at the Farmer's Market in the morning (see Cheap Date Idea #50). Or come later in the day to enjoy the sunset by jumping on the ferris wheel or better yet for free by laying out on the beach.
If you love ocean life, check out the Roundhouse Aquarium at the Manhattan Beach Pier for FREE every day of the year (Cheap Date Idea #6).
Approximate cost of creative date idea = $2-5 Per Person + Parking.
Actual value of the cheap date idea = $4-10 Total.