There's nothing a computer nut loves more than a new computer gadget. Whether hardware or software, computer gifts can be as simple or extravagant as you choose. Here's what we're getting for our happy high-techers.

For $70, we're going to organize son Jeff's time so he never misses another appointment. We'll get him the program On Time. Subtitled The Calendar that Means Business, it sure does. Onscreen, On Time shows two calendars, one for appointments, the other for non-appointments.It prints out monthly or daily calendars.

It keeps track of events, such as birthdays and anniversaries, that recur every year. It also tracks one-time, weekly, biweekly, monthly, quarterly and half-yearly meetings. The program works only on IBM compatibles, but Jeff's Amiga has an IBM emulation board.

On Time arrives with four categories of databases: appointments and meetings, work tasks to do, personal tasks and anniversaries. Jeff can add up to 26 more categories. If we know him, there'll soon be one database apiece for his bride, his cat, his canary and his iguana.

Besides being flexible, the program is fast and easy to use. To enter an appointment, Jeff has to hit just four keys. For each reminder he enters, he can type in a line or two of information. If he forgets what date we're arriving for a visit, he can search the date book for our names.

Best of all, it has an appointment beeper. No matter what he's doing, if he's doing it at the computer he'll hear the reminding beep. (Campbell Services: phone (313) 559-5955.) For Joe, who's a careful planner, we'll get $100 PercentEdge. It can help him figure out where to put the extra money. If he knows the best interest rate he can get, the life of the loan and the down payment, it can tell him in seconds the highest price he should pay for a new car. There are pop-up help screens that explain financial basics.

We used our copy to find out how long it would take to become millionaires depositing $1,000 a month into an account paying 8 percent interest. PercentEdge's Variable Rate screen told us it'd take 25 1/2 years.

The program fills in amortization schedules for loans, mortgages and similar money-in or money-out transactions, including loans that provide for balloon payments. It calculates savings balances and annuity values. It compares CD rates to show which plan is best and handles variable interest rates.

The program can be used either pop-up or stand-alone. It includes a pop-up calendar and a calculator with a memory. If local dealers don't stock PercentEdge, phone (800) 338-5314, in MA (508) 768-6100.

Son David could do with some fun to break up the college grind. We'll get him a mouse named Felix for the holidays. Made for Mac computers, it's 5 by 5 inches square, small enough to fit on Dave's student desk. He can plug it into any mouse port. It's fingertip-operated: small finger motions take the place of the arm movements that run most mouses.

The software that comes with Felix includes a bunch of cute tricks. Dave can command the pointer to go right to a window's open-close box, zoom box or scroll bar. In using drawing software, he can set the mouse to move slowly in a small area and faster over wider space. A Paint-and-Signature setting permits very fine arrow control.

Here's a trick Dave will find useful in writing school papers. If he opens a new window, Felix clones a new arrow. The old one saves Dave's place in the old window while the new one is completely autonomous. At $170, Felix is priced like most mouses. Its maker, Altra, claims it lasts five to ten times longer.

We'll see. (Altra: (800) 726-6153) Daughter Alicia's getting Seiko's Smart Label Printer. Her last excuse for not writing was that her computer printer doesn't print envelopes.

SLP takes up less room on the desk than a small telephone and connects to any IBM or Macintosh type computer. It holds an adding-machine type roll of self-adhesive labels. Its memoryresident software lets anyone use it right from almost any word processing program.

Here's why it's smart: Once it's set up, the software can find any name-and-address series of lines on the computer screen. It draws a rectangle around those lines, takes a quick snapshot of what's inside, and sends it on to the label maker.

Labels are roughly an inch by 3.5 inches and stick to any envelope or package. Before printing, you can adjust lines and change spellings.

The printer doesn't just type names and addresses. It can print out warnings like FRAGILE and slogans like "Use recycled paper." There are three sizes of type to choose from in two styles, serif and sans serif.

The printout isn't anything to write home about. It compares to what you get in fast mode from a cheap dot matrix printer. But SLP also prints bar code equivalents for numbers. Alicia will be able to make good use of that when she opens her own audiology clinic.

The label printer and 130 labels list for $250, with extra rolls $6 apiece, but some office supply stores discount it. Seiko: (800) 553-5312; in CA (800) 553-5315.