In addition to Senior Thesis research, many students take part in research opportunitites during summers, or during academic years prior to senior year. Starting early on research projects can be a great way to learn more about different areas of physics and gain experience in a variety of experimental or computational techniques.
The Undergraduate Research Opportunities Program (UROP) web site is a great resource for finding research possibilities at UCI and elsewhere. UROP maintains a list of faculty who have undergraduate research openings in their groups, but it's important to be aware that this is a very incomplete list: many Physics faculty will have openings that aren't listed on the UROP site. The best way to find research opportunities is to contact faculty directly, regardless of whether they have an advertised opening.
The UROP program has an annual call for proposals for research grants that can be used to help fund student research projects. UROP also sponsors various other undergraduate research fellowships, which are listed on their web site.